Besides the various individual activities of the city's villainous types, and the smaller activities that a patrolling hero might encounter, a good sandbox superhero campaign will also have larger events occurring at the same time that can also be of interest to the city's heroes. Such events can be similar in nature to the sort of things that a hero might encounter while on patrol, but will usually be larger in nature and scope, the sort of thing that the entire team could try to deal with; the former might involve an office building on fire with a half-dozen employees at risk, while the latter could be a high-rise building engulfed in flames with dozens of inhabitants threatened.
The Citywide Events table covers this sort of thing. How frequently it should be used is campaign specific; once a week, twice a month, or monthly are all possible options; a good rule of thumb is to have said frequency roughly coincide with how much game time passes between gaming sessions (this should ideally be the same as the time block you use to determine activity by gangs, organizations, solo and group supervillains). Whatever the frequency, roll 1d3-1 times on the table below for each block of time that passes during the campaign. Such events can be caused by, or tie into, the various criminal NPC's already chosen for the game, but they don't always have to be. Note that, due to the flexibility of any given event, two completely different entries could yield similar results - this is deliberate.
(and of course, even if you have no desire to run a sandbox game, these tables can still be used to generate random scenario ideas for a more traditional superhero game)
CITYWIDE EVENTS (ROLL D10)
1-2 Organizations, Gangs & Government 3-4 Solo Supervillains 5-6 Supervillain Groups 7 NPC Superheroes 8 PC's & Friends 9 Other Crimes 10 Weird & Unusual StuffThen roll on the indicated sub-table below.
ORGANIZATIONS, GANGS & GOVERNMENT (ROLL D10) 1 Organization recruits supervillain (even odds as to whether this is a temporary or permanent addition) 2 Supervillain defects from organization (they probably don't take this very well) 3 Two organizations come into conflict (could be loud and messy, could be quiet and occurring in the shadows) 4 Internal power play in organization, coup d'état (could play out fairly quickly, or could be a protracted civil war - however long it takes, the head of the losing side, assuming they're still alive, is probably now on the run from said organization) 5 Temporary alliance between street gang and supervillain (the villain has a temporary use for the gang; whatever happens, it probably won't end well for the gang) 6 Two street gangs come into conflict (innocents are no doubt hurt as they get caught in the crossfire) 7 Terrorist activity (could be either foreign or domestic terrorists, possibly superhuman) 8 Government group goes on strike (could be a group that affects the public safety, like police or firefighters, in which case the heroes may need to pick up the slack; alternatively, could be an organization of less obvious impact, which might have a more subtle and personal effect on one or more of the PC's, or it just might be brought up for more humorous effect - sanitation workers go on strike, and the garbage keeps piling up outside the PC's base) 9 Local government infiltrated (probably an organization taking control, although shapeshifting supervillains are also a possibility - will almost certainly make things more difficult for the PC's 10 Diplomatic visitor (possibly an elected official from another city, or perhaps from another country, and needs to be guarded from threats - optionally, not an elected official, but a wealthy (and probably obnoxious) celebrity whose presence makes things difficult for the PC's
SOLO SUPERVILLAINS (ROLL D12) 1 New villain arrives in town (might be here for a one-off job, or might be looking to make the city their new home) 2 Two villains team up (could be a one-time thing for a specific task, or could lead to a partnership - if the latter, the duo rolls a d8 instead of a d6 to see if they are active during a specific time-period in the campaign) 3 A new supervillain appears 4 Two supervillains fight each other 5 An incarcerated villain manages to break out of jail 6 An old villain returns (might be returning from the dead, or just coming out of retirement) 7 Villain seeks aid from hero or hero group (might be pulling a scam, or might genuinely require aid for a good cause that only the heroes can provide) 8 Villain calls out hero for fight (possibly a trap, of course, but done with enough publicity that the hero looks bad if they don't show up) 9 Villain gets on bad side of organization (might become a temporary ally of the heroes, depending on their feelings toward the organization) 10 Villain switches sides (could be part of a scam, might be a legitimate face-turn, although even if genuine the villain may eventually backslide into old habits) 11 Temporary alliance of multiple solo villains (probably to pull off some big score, Ocean's 11 style) 12 Villain pretends to be hero (probably to make the hero look bad, although possibly just to mess with their minds)
SUPERVILLAIN GROUPS (ROLL D8) 1 Villain group arrives from out of town (probably just for a temporary task, but there's always a chance they might want to relocate on a more permanent basis) 2 Villain group recruits one of the local solo villains into their ranks (could be temporary, hiring them for a specific task, or they could be a permanent addition) 3 Villain group loses one of their members (could be a mutual decision by both sides, but probably not) 4 Villain group attacks a local solo villain (for pretty much any reason, really) 5 Two supervillain groups fight each other (the potential for innocent casualties and mass destruction should be huge here) 6 Villain group decides to target and attack the PC's (could be a simple ambush, or might take some time studying the heroes to make sure to exploit any weaknesses) 7 Supervillain group clashes with an organization (the heroes may not know which side to support, if either, but things might get messy enough that the heroes have to do something) 8 Power play withing supervillain group (someone tries to be the new boss - the loser of such a struggle may or may not be ousted from the group, depending on the egos involved)
NPC SUPERHEROES (ROLL D4) 1 New superhero shows up (may want to join with the PC's, but is obnoxious/incompetent enough that the PC's probably don't want to work with them - alternatively, the new hero may choose to avoid working with the PC's, for various reasons) 2 Superhero shows up from another city (probably on a temporary basis, although they might be setting up operation on a more permanent basis - may or may not seek to work with the PC's) 3 Hero group shows up from another city (may not want to work with the PC's, may even have to come into conflict with them) 4 One or more local NPC heroes goes missing (the PC's, of course, will have to investigate)
PC's & FRIENDS (ROLL D6) 1 Player Character targeted for romance (this could be in either their secret or heroic identity) 2 PC's secret identity is targeted (PC may become aware of someone snooping, or someone may have already discovered the truth and is threatening/blackmailing them - alternatively, might not be someone hostile to the PC, but instead a friend or family member) 3 Friendly NPC threatened (anything from a stalking ex-spouse to local gang members to a supervillain who wants to win their affections) 4 PC's secret identity gets attention (their 15 minutes of fame, for whatever reason, not connected to their heroic identity, although the media attention may make slipping into their heroic guises unnoticed more difficult - if the PC doesn't have a dual identity, they get media attention for something besides their superheroics) 5 Unusual will request (could be from anyone, from along-lost relative, to a NPC hero or even a villain - must perform some unusual deed or finish up some long unfinished bit of business - alternatively, might inherit an unusual device or odd bit of property (great for heroes without a base, especially if the location is haunted or otherwise quirky) 6 Friendly NPC or NPC's get mutated/altered in some way (probably temporary, but always a chance that there might be more permanent affects)
OTHER CRIMES (ROLL D8) 1 New designer drug show up in town (could just be the latest variation of crack or meth, or might also give the user temporary super-abilities) 2 Multiple super-villains break out from the local super-prison at the same time 3 Kidnapping (possible high-profile enough that the PC's are called in to help, or perhaps a friendly NPC is abducted) 4 Public official targeted for assassination (the PC's have to provide protection, of course - the target might for good measure dislike/hate the PC's) 5 Influx of guns/other weapons in the area (might just be a proliferation of real-world weaponry, or high-tech stuff might somehow be finding their way into the hands of local organizations and/or street gangs) 6 Citizens endangered (anything from an arsonist setting fire to a high-rise building to a lunatic threatening to poison the city's water supply) 7 Robbery (possibly a heist audacious enough that it gets lots of media attention, or could be a local/friendly NPC whose livelihood is being targeted for some reason) 8 Innocents targeted (someone is targeting victims based on some ethnic/creed/gender/profession/etc classification, i.e. Hispanics, gays, Catholics, prostitutes, veterans, children, etc - may be killing them, or possibly kidnapping - probably for prejudicial reasons, or might may be using them for something more esoteric, such as to complete a magic ritual)
WEIRD & UNUSUAL STUFF (ROLL D20) 1 Unusual weather (could be something natural, depending on the location and time of year, such as hurricanes, tornadoes, forest fires, etc, or could be something caused by a supervillain or some unusual artifact) 2 Extra-dimensional being(s) shows up (someone or something from a non-earthlike dimension, could be an advance scout for an invasion force, could be fleeing some terrible oppression, could be looking for a new home, could just be lost) 3 Alien(s) show up (motives similar to extra-dimensional beings as above, may or may not have a starship with them) 4 Mass supernatural occurrences (outbreaks of vampirism, people seeing ghosts everywhere, people unknowingly casting spells based on their words or wishes, etc) 5 Dimensional rift opens (could be to another world, another dimension, another time, etc. - probably temporary, but a slim chance it might become permanent) 6 Temporary boom in superhuman activity (people gaining temporary superpowers, for both good and ill, with the chance that for some it might trigger a more permanent change - optionally, it doesn't create new superhumans, but rather something compels everyone with unusual abilities, including those who normally choose a more mundane lifestyle, to become more visibly active during this time period) 7 Science runs amok (any super-tech characters or organizations might do something inadvertently affects the area, such as creating an intelligent computer virus, temporary time-shifts, subsonic emissions affecting people's personalities, etc - this can also include more mundane failures of science, such as a citywide power blackout) 8 Space-related disaster (meteorites crashing into the city, space station astronauts needing to be rescued, etc) 9 Someone from a parallel universe shows up (could be refugees, invaders, multi-versal tourists, etc, may or may not have a way home, and for good measure could also be analogues of the PC's) 10 Time traveler(s) shows up (could be from either the past or future, either controlled or accidental - if from the future they are probably trying to prevent/ensure some event from occurring, which may put them in conflict with the PC's, if from the past they may be seeking insight into their own futures, or may be wishing to acquire some advanced piece of technology) 11 Prophecy (the PC's get some warning/vision of the future, and have to act accordingly - optionally, a NPC has to deal with the prophecy instead, and as a result is acting somewhat contrary to their usual nature) 12 Mass mind control (someone or something is controlling/ influencing the local populace in some malign way) 13 Mass transformations (could be caused by an alien virus, technobabble rays, demonic energies, etc, possibly enacted by a mad scientist or alien invaders, is probably temporary/ reversible, but may trigger more permanent changes in a few people) 14 Rampaging monster (said monster will almost certainly have high strength, nearly invulnerable to damage, probably isn't too bright, and optionally may be several stories tall) 15 Mass amnesia (should ideally affect the local super-types as well - PC's can have fun trying to re-learn their abilities, and villains can discover if their criminal behavior is more a matter of nature or nurture) 16 Animal life goes nuts (possibly being controlled by some supervillain, or perhaps some alien/mystical artifact is causing the local wildlife to go wacky) 17 Social unrest (tensions flare over anything from racial strife, gay-bashings, unpopular courtroom decisions or religious/ political scandals - enough so that there will be riots/violence in the streets) 18 Media smear (the PC's - or perhaps someone close to them - takes a drubbing in the press ("threat or menace?") 19 Religious/cult activity (unusual or controversial edicts from a major religion, possibly directed at or concerning the PC's - probably hostile, but also perhaps venerating a PC or PC's - or perhaps a smaller cult makes things interesting, either through malign or merely secretive/unusual activities) 20 Superhuman tournament/competition (some sort of contest for a grand prize, a la The Great Super-Villain Contest, could be more specialized, such as a martial arts tournament, or might just be seedy and sleazy, like an underground pit-fighting broadcast)
(to be concluded... eventually)