Tightly wrap saran wrap around the bowl of the toilet so that no folds are visible, and then lower the toilet seat. The next user will find their human waste not entering the toilet. See it in action here.
Fill a bowl with water, stand on a chair, and place the bowl directly on the ceiling. Push a broom stick on the bottom of the bowl to keep it there. Ask someone to "just do a quick favor and hold it there for a second", then take the chair away and they are left there with no way out but to let the bowl of water fall on them. See it in action here.
Freeze a can of shaving cream, then hacksaw off the bottom of the can and put it in a desk. The cream will expand while thawing a very large amount. See it in action here.
Put dish washing detergent in the toilet so that when it is flushed, bubbles will emerge from toilet bowl.
Take the center out of each Oreo in a package and fill with tooth paste and freeze over night. See it in action here.
The classic of squirting whipped cream or shaving cream in a sleeping victims hand and then tickle nose with feather or hair and they will mash the cream in their face. See it in action here.
While your victim is sleeping, quietly place a small coffee table on top of sleeping person so that there are no legs touching the body. Make sure the persons head is directly under the table. Then blow a fog horn and the sleeping person will jolt up and ram their head on the bottom of the table. See it in action here.
Take of the top of your toilet. Using strong tape, take the tube that refills the back of the toilet container and redirect it so it is at the rim of the container. Tape the tube so that it points outward towards the toilet bowl. Replace top. The next person who uses the toilet will flush and spray themselves. See it in action here.
If you're at a beach, choose a victim, and when they get up to go to the bathroom dig a deep hole where they were sitting and put the towel back over it so when they return they will fall into a trap.
This one is sort of like the ceiling bowl one, but you bet your friend that he/she can not balance two glasses of liquid on the backs of their hands for a certain amount of time. Place the glasses on the backs of their hands, (on a table) and then leave them to deal with that.