Well, if the object is to do something to benefit mankind, we could do these things:

- go back in time and slip a contraceptive pill into Mrs Hitler's tea (or do likewise for your dictator of choice)

- go back in time and destroy the blueprints for the internal combustion engine, thereby saving the world from global warming

- go back in time and get loads of dosh as mentioned for mersonal use in posts above, but bring it back to the present and use it for charitable works to improve the lot of poor people worldwide

- go back in time and persuade everybody that you are their god, and that your dearest wish is that they will never start fights on your behalf, and that world peace is a central tenet of your/their religion.