the upside down cup of water trick on the desk is funny
but the best one is a snake stratigically placed somewhere (glove box, fall's when door opens, in the mail box... etc...) I put one of them 4 ft look alike rubber rattle snakes under my pop's trash the night before he takes it out to the curb. Put a couple bottle caps on top of the bag the snake was under so when he picked up the back, the tops would fall and get his attention, then he'd see the snake.
We'll it worked too good!!! He smacked that snake twice with the shovel before he realized it was fake... I cant express in words how hard we were laughin!
Spraed some catfish bait under the draw on his desk. I would advise clearing the area or remove all people who can not stand cussing. We did that to a police officer car door handle the words that came out of his mouth would make a ******* blush