Dig under your buddy's house while crouching, proceed to build a redstone pulser and then link it to ten wooden doors. The rage to joy ratio was extremely high until he found it. It's an old, old trick, but it's still so damn worth it because they literally have no clue what's going on.
Edit: for maximum trolling, hook the pulser to a lever that is actually inside the house. That way, he/she can turn it on and off, but if he/she breaks the lever, it stays on permanently.
Edit: for maximum trolling, hook the pulser to a lever that is actually inside the house. That way, he/she can turn it on and off, but if he/she breaks the lever, it stays on permanently.
Edit 2: kittens are quite nice.