Probably what all other ghosts do assuming they are real - try to figure out how to move on while inadvertently haunting others (maybe calling out for help?). I can't imagine being a ghost would be very fun for long. Existing in such a state would be pointless if your actions had near zero consequences.
At least in lucid dreams you have control over the world around you and can have fun. But physical reality where the most you could do is drop the temperature a few degrees in a room? Pass.
Even if you could do that, it could still be used as a way of encoding information and communicating with someone.
If you got really good at it, you could quickly alter the temperature in a small area and quickly convey information that way.
Or else get a job as an air-conditioning system. I don't know how they'd get paid, or what they'd buy.
I guess a computer, Internet access, and Netflix?
(Which raises yet another question about the physics of ghosts: If they can lower the temperature of a room, what is the mechanism used? Are they serving as a heat pump? Where does the energy go?)