I'd probably haunt people who are majorly outspoken athiests, and pretend to be god, why? Cuz it's funner to fuck with people who don't think it's possible, than people who are looking for it.
Might just like, choose some medium or whatever and completely destroy their credibility somehow.
Maybe it's because I don't generally like people all that much, but I'm sure I'd do some nice things, like maybe finding a kid who's being bullied, and proceed to solve the problem one way or the other, either helping the bullies not be bullies anymore through suggestive measures, or just break their minds with a constant campaign of psychological warfare.