Depending on who the prank is for, I read one a few years ago out of a Captain Underpants book that I have been wanting to execute ever since. Take 2 ketchup packets (like the kind from McDonalds) and put one under each of the little humps under the toilet seat, when your husband sits down...Squirt! ha ha ha
A little too much? Better than celophane under the toilet seat, right?