I've read about some bacterias or viruses before, about some very resilient life form that could withstand any known conditions, and which is practically immortal... OK, I see someone mentioned them already. Tardigrade, yes. Generally simpler life forms sounds like fun. You do small stuff, and then you die.
Or maybe reborn as a human with a rare physiological condition which severely inhibits pain and fear. Or a human with an inbuilt killswitch, so that I could do anything without worrying too much about the consequences.