The bathroom may be the best room for pranks. A lot happens in there--gossip, beautifying, going toilet, alone time, et al. Needless to say, since I just said it, the bathroom is a great location to pull some pranks. May the following inspire you and unsettle those just trying to relieve themselves.
You're in the bathroom (at work, school, or a public restroom) and you hear that there's only you and one other person in a neighboring stall. Quietly, you start to sing "I think we're alone now / There doesn't seem to be anyone around."
Replace the toilet paper with party streamers.
Replace the hand soap with fake or real blood if it is a pump or with a rectangularly cut potato if it is bar soap. Potato or blood hands!
Put fish, seaweed, and sand in each toilet bowl or release a colony of bats into the bathroom. Do not do both; this world is cruel enough as it is.
Install huge video cameras from the early 1990s inside the stalls. People will have to hold it if they don't want to be filmed, and they may not want to be.