Hearts & Souls, superhero games are easy enough, in general. Make some characters, go fight crime. While it doesn't have to be a literal fight either. Could be investigation into a kidnapping, the stopping of a flood, the rescue of people trrapped in a burning building. All the usual fun challenges superheroes face. Throw one out, let the heroes go after it.
Other similar superhero games with low prep: Truth & Justice, Bash, 4C/MSH, Marvel Saga, Supercrew.