If I could know then what I know now, I would go back to my freshman year in college, so I could make different choices and change the course of my life. Being past 50 is a fabulous blessing, because of all the wisdom I have acquired. I wish I'd had myself for an older sister, to give me guidance in my younger years!
As for time travel, there are too many historical periods to count. My favorite fantasy is to go visit the world of Laura Ingalls Wilder. In the book "The Long Winter", I could show up in a helicopter at a crucial time, and deliver coal and canned foods to the town of De Smet. Imagine the impression that would make!