I would go back to 1750 and start working on our founding fathers. I would beg, bully, cheat and plead them to not allow slavery into the republic they would build.
Obviously changing some atrocity that happened in the past seems like a good thing to do, but what if preventing that act or event actually causes mankind to be worse off in the future?
Easy, I would buy Microsoft and Apple stock in the 80's! . The only logical move IMO is to better ones self rather than "play God" with the lives of others.
It should be against the rules to interfere with history of the world if time travel were a reality. For example, removing Hitler may have resulted in Stalinism/communism spreading over all of Europe. That was Hitler's biggest fear after all--many don't know this, but Jews and Communists were practically one in the same to Hitler (thanks to the Bolsheviks/Trotskiites). Oh yea, and if WW2 never happened, you most likely would never be born because the Baby Boom wouldn't have occurred!
Imagine if the US did not invade Iraq. The Arab Spring would most likely never have happened, and the status quo of poverty and oppression in the Middle East would continue to churn out suicide bombers.