Music games. DDR, PIU *machines or bust*, Jubeat on iPads even.
Fighting Games: UMvC3, TTT2, P4A, SFxT, Brawl, Injustice, whatever else can be wrangled up.
Classic co op games: *streets of rage, simpsons arcade, ninja turtles, etc
Shooters: Because everyone and their mother plays CoD and friends..
Tournaments for as many things as possible. It's not hard to organize, run, and give small anime/con prizes for.