Hmmm....
First, I'd go back in time (okay, if we can pretend we can turn invisible we can pretend other things too!) and spy on my ex-boyfriend who cheated on me for the entire relationship. I'd videotape him and then confront him and make it a bigger and louder and more embarassing confrontation than it was to begin with (no, I'm not still bitter!).
I'd sneak over to Hollywood and photograph celebreties doing mundane things (or weddings, whatever), then sell them to magazines and make my fortune.
I'd go on a huge shopping spree in the new Nordstrom and Anthropologie (among other stores) in the SouthPark mall.
When election time came I'd head to the polling places and change people's votes so my guy could win.
I'd definitely mess w/ people...like move their shirt to another side of the room, hide their towel while they were in the shower, etc., so they would think they were going crazy.
I'd test drive cars and turn invisible to freak out everyone else in traffic next to me.
And finally, I'd spy on my office manager at work while she was surfing the net and playing solitaire to use as ammunition in case I get yelled at for surfing GC all day.
And of course I would spy on hot boys...while they were working out....w/ no shirt on.