My favourite harmless prank I've heard of was done by this girl whose dad was a geologist, and they'd go on day hikes with his geologist friends/co-workers and when she got bored on them she'd habitually pick up a random rock and go ask him what it is, and one of them would explain what kind of a rock that is, how it probably got here, and usually some notions of the more unusual features the rock had, if any.
And she had a friend who had once gone on a tourist trip to Iceland and brought back a volcanic rock. So she borrowed the rock and took it with her on the hike, and after two randomly picked up "hey dad what rock is this", she presented the volcanic rock, in the same fashion as all the others.
3 minutes later there are five middle-aged and older men circled around this mysterious rock, all agreeing on what it is, but not why it is. They keep asking her questions, where did she find it? Were there any other rocks around there that looked like it? Was it like this on the ground? People walking past the group try to stretch their necks to see over the geologists' shoulders to see what's the source of such amazement.
And in the end she couldn't take it anymore, burst into laughter and confessed. The geologists agree that it was pretty clever.
