I took a packet of catsup, folded it in half and then place it under the toilet seat, right at the spot that the little knob that keeps the seat elevated from the bowl. When the next person sat down the packet explodes onto their leg.
My son tells me they still do this in school.
I used to work as a cashier and we used to play pranks on each other all the time. Once someone put a peeled banana in my purse and didn't tell me... quite a surprise when I found it the next morning. To retaliate I took the guys gloves and crumbled pretzels into them.