Haunt some of the powerful but evil people in the world and see what I can change. Let’s see how effective a dictator like Kim Jong Il can be with a ghost all up in their business.
If I have poltergeist abilities then I suppose just straight up assassinating people would be possible. After all, if you can shove a book off the shelf or flip a light switch then breaking a few tiny blood vessels in someone’s brain to give them a stroke is going to be easy. Not that killing other people would be easy, but maybe as a ghost I’d have a different perspective on death.