Wide plastic packing tape wrapped so that the sprayers on the sink faucet were "on" all the time. I pulled that one on my SO's dad. He got up to make coffee in the morning and got a shower instead. He most delicately snuck into my room and sewed one of the legs of my jeans shut at the knee and then hit the fire alarm....gawd I love that guy.

Plastic rodents and reptiles are great. Not as earth-shattering as the real thing, but will do in a pinch.

Bucket of ice water dumped on a showerer will illicit screams of such magnitude as seldom heard on this planet.

But there is alot to be said for the extended variety....One summer our entire family went camping in Eastern Michigan. I came into the cabin and my Nana and Auntie stopped talking immediately. I, of course, BEGGED to know what they were talking about. No, they can't say...we're too young....well...they'd tell me, but I had to PROMISE not to tell my cousins (fat chance).

They told me that this cabin was haunted. In the 30's, a man had killed his wife and her lover while they were making it right on THAT couch (the couch where I was sleeping, o'course) they had feared for the family, sure, but it was still a nice cabin....I ran out and told everyone, but it was a secret...

This was followed by TWO WEEKS of my grandfather scratching the walls of the cabin in the night...my uncles letting out mysterious moanings....my dad wandering around outside with a sheet over his head....and about 15 kids that got absolutely no sleep whatsoever...gawd I love those guys...