Here's a good one I pull every year. All it requires is a drinking straw and a toilet. No not what your thinking. Take the fill tower lid off the toilet. Disconnect the actual filler tower tube that returns water to the fill the tank(only disconnect the end that fills the reservoir tank). insert one end of the straw into the disconnected end of the filler tube and lay it down over the tanks edge so that the other end is pointing at a would be flusher. Replace the fill tower tank lid. This will help the straw stay in position. Now that the trap is set! Just wait for the next unsuspecting flusher, and let the laughs begin. To undo the prank simply remove the straw, and plug the tube back into the top of the filler tower so the tank will refill normally.