Oh, you know... spiking the punch. Taping signs to people's backs. Saran wrapping the toilet seats.
For sleepovers, there's the classic hand in a glass of water or wet willy. I was never good at the drawing on the face thing. But, put a dollop of whipped cream in someone's hand and tickle their nose with a feather, and that'll get a laugh from quite a few crowds.
[Ideas? What ideas? Duo's just being encouraging!]