I love gag stuff. I love to have gags pulled on me, too.
Subtle ones are my favorite. I like to have those clocks that run counter-clockwise and the numbers arranged backward. Sometimes I ask people what time it is, and I hear a weird note in their voice, and they'll say, "Something's weird about that clock, but it keeps good time. I can't put my finger on it; what's going on with it?"