One of the best pranks I saw involved a house sitter, and some musical Christmas socks. Before going away, they removed the music module from an old pair of Christmas socks that played "jingle bells". They then put it between the mattress and the box frame of the bed a relative would be using, while house-sitting. When they turned over in the night, they would set it off. They had jingle bells stuck in their head for a week, but didn't know why.

On another note, a work colleague is the master of ear worm deployment. He has the knack for humming catchy tunes just right. Just a few bars, quite enough you don't notice. You suddenly find you have a tune stuck in your head. You know what he's done when you realise that half a dozen others also keep murmuring the same tune (it's stuck in their heads too). It's remarkably self reinforcing. It amuses him greatly , while being fun and non-harmful.