Here's one that's really harmless.
You go outside with a piece of string (string is good for lots of pranks). Anyway, stand on one side of a house or building and wait for a sucker (another word for friend). Now, get them to hold one end for some concocted project, while you disappear with the string around the corner and wait for another friend (another word for sucker) to happen along. Talk them into holding that end for some concocted project or other. Go hide, and see how long you can have them standing there, for no reason at all.
This goes back to two salesmen calling on an office in which I worked. The first appropriated the desk and chair of our rig superintendent. The other dropped in later, introduced himself and sat in the visitor's chair. Whatever they were selling, they tried for ten minutes to sell it to each other. The rest of us just watched, looking like a couple of fat frogs on lily pads.