If you're not familiar with the book "What if Everybody Did That?" by Ellen Javernick it is a book that repeats that question on every page after explaining a situation where someone is breaking a rule and showing the impact it has on those around the rule breaker. It's got a silly, fun vibe, but makes the kids think. I highly recommend reading that book before doing this activity, but you could still do this activity without reading it at all.
The idea is flipped here. Students think of good things people can do that would make a positive impact on the people around them.