Hmm. I could easily answer meerkat to the animal idea, but I don’t know if meerkats actually lead easy lives! The obvious animal answers are ‘cat’ and ‘dog’, because the ones I know usually have relaxed, stress-free lives!
As for ‘thing’, I don’t think I’d want to come back as an object, so we’ll nix that one completely.
As for person, well, if I could inhabit the life of a famous and successful F1 driver, or a successful footballer, I’d be happy. Some trophies, a good bank balance, it’s all good. I’d prefer to live a life as me achieving those things, but if I had to live out a life as say, Lionel Messi, or Sir Lewis Hamilton, so be it.
Sometimes though, it’s the simpler ideas that appeal. If I could come back as a more successful version of me, then great! How about a life where I’d written my book a few years sooner, or generally been better at any given job? A life as a successful author would be nice. If the opportunity exists to experience different lives for myself, I’d be fascinated to see how that goes, as long as I could experience them with my wife and daughter.