Honestly I think I would have a lot of fun haunting dreams and such and to become like an urban legend. Things such asscaring the living shit out of multiple people by dying in front of them in their daily livesand then making them think it was a dream.
RE: What would you do if you were an omniscient being you could fuck around and play god.
Whenever I would hear someone say the phrases "What could possibly go wrong" or "At least it can't get any worse" , I would happily show them the endless possibilities.