One prank I haven’t seen listed yet is the one I used to do at summer camp and the college dorm. Take the top off the toilet tank. Inside, there is usually a vertical plastic pipe about 1 inch in diameter.
Going into the top of this pipe is a little plastic tube. Turn the tube outward and, if it is long enough, then put it toward the toilet bowl with the end just sticking out. Replace the tank cover, making sure that the little plastic tube is just sticking out. When someone flushes, the tube will squirt water.
One time in Colorado I did this at 3:00am. The guy that got caught must have flushed with his elbow while still seated. His back was sprayed with ice cold water. His language was abominable, and made funnier since this was a Christian camp. Oh well, we’re all human.