What would a good college-style prank be?

Some girls went and post-it-ed some of the guys cars, and put a bunch of other stuff on our doors, so we are all getting together to get them back. 2 of them live together, but we want to get all of them back. What would be a good, clever (somewhat appropriate) prank to get them back with. Please don't mention the obvious - saran wrap, peanut er on door handles, ect.