The thing is, even if you could go back, nothing would change. Whatever you change will alreajdy have happened from the future you going in the past on the first place. It is a never ending cycle if one were to think about it.
Or you have a paradox because if you go back to kill you Grandfather ( just hypothetical ) you will never be born. But since you were never born, your grandfather couldn't be killed,.
But if I could, and just observe, I would go back to see Jesus' ministry, as well as the signings of very important documents and treaties that changed the world. Oh, and the first day Disneyworld and Land opened.