The problem with a concept like this is that it's all hypothetical. it could never happen. Machine or not, you ain't ever going anywhere other than your own personal moment. People just don't really understand the nature of time. Most importantly, the fact that it doesn't exist. At least, not in the way people choose to comprehend it, anyway. Time is nothing but a creation we've come up with so our little primate brains can wrap around the concept that there's no "Now". It's the way we can understand the distance between not places, but events.
Ok, that being said...I'd probably leave most things alone, unless it was something personal. I know the big thing is to usually say that you'd go back and kill Hitler or something. You honestly think that would have stopped anything? It would have just postponed it. I know it's a rather banal example, but think of John Connor in Terminator 3. "But...but....we stopped Judgement Day!" "No, you just postponed it...." Dumb example, but it's true. Stopping Hitler would have done nothing but hand the reigns to someone else with the foresight to take control of what was already going on.
Hitler did not just arm himself up like Rambo and walk into Poland, or single-handedly make Eurpoe lie down. He had an entire nation's military behind him. He had an entire (constantly growing) culture behind him. People who agreed with him and completely thought they were doing the right thing. This is a movement that is not the creation of one man. So often when looking back on history, we focus on the person in charge, and not the social stream of the time. There was a reason Hitler was able to take over an entire country. It was a social shift, a direction of humanity that was going to happen no matter what you do about it. There is ZERO chance you could "go back in time" and prevent Nazi Germany. You'd only postpone it, or perhaps even inadvertantly strengthen it. Things happen for a multitude of reasons, not just because one person did something, or one event changed everything.
Think of the assassination of Archduke Ferdinand. Kicked off World War I. Yeah, officially. Think you're gonna stop World War I by stopping his assassination? M'kay.....go ahead. The machine was in place. The emotions were there. it's one small thing leading to another small thing to creating a bigger thing that joins with another bigger thing which is itself a combination of smaller things. Major shifts or streams in human development and social events are going to happen no matter what you do.
All of that being said, I'd probably just go back and re-experience something really great that I know has already happened.