Harmlessly sadistic, of course. Here are a few things I like to do to mess with people's heads while on the street.
1) When making eye-contact with a stranger on the street. Flash them a smirk like you know something that they don't. Puzzled looks guaranteed.
2) Someone walking too close behind you? Walk fast, slow down, walk fast, then slow down again until they pass you.
3) Start laughing out loud when you think of something funny (as in, don't laugh inside to yourself).
4) When getting in someone's way, move aside in the same direction they are a few times (think getting in the way of a running back). Then stop and part your arms as if you're parting a way for them...make sure to flash a big, fake smile.
5) Stop short.
6) Look up, then look around like something is in the air...that always gets at least one person thinking they should be looking for something.