A prank I played on my roommates (I had three of them at the time), was putting shaving cream in their towels (which were hanging up in the bathroom). However rather than not putting any in mine, I did mine and didn't do someone else's to make the illusion that he did the prank. It worked quite well. One roommate showered first, got full of shaving cream, noticed the one towel without shaving cream, got pissed, then went to that person's room and sprayed shaving cream all over the place. It was quite funny.