I'd wait till a guy or some guys are walking through a mall or down a street behind a girl, and slap her butt, making her think it was the guys. That would be soooo much fun. I;d also sneak into houses and scare people. Id walk around naked too, I mean no one can see me right? I like a healthy breeze.
I would fight bad guys and MAYBE spy on some hot actresses while they are changing.
I could try and break into NASA....
and assuming that I could become visable whenever I want, I would wait for people I don't like, punch em in the face or trip them and when I turn a corner, become 'visable'. That way my enemies would blame me, and if we got in a fight, everyone else would beleive they struck first.
or sit on a semi full bus and shout 'this mans gotta gun!' or 'Theres a bomb on the bus!' Good stuff.