If anyone around is a Merry Prankster, post up on what you like to do to fuck with tripping people (but not to ruin their lives, cause that's just horrible). My favorite is to whisper nonsense at a barely audible level, and as soon as anyone notices, stop and act nonchalantly. Then, start again when the trippers stop looking around for the source of the sound. It's amusing to watch the tripping people get quite confused.
One thing thats funny is wiggling your fingers at the person and making weird noises. My friends did that to me and i thought it was great, kinda spooky but fun.
This is not so much a prank as it is a visual-inducer is to place your hands on your face and vigorously rub it with pressure (allowing the fingers to sink into the skin of the cheeks). It makes it seriously look like your face and hands are melting together. This made my girlfriend cry one time when a friend did it to her. I was amused.
When someone blows a fake bubble, making the hands look like your holding a bubble, and start talking then act like your putting the bubble over their head and keep mouthing words but stop talking. I thought that shit was fuckin hilarious when i was trippin. And the wiggle hands and stuff...