I had to put a bit of thought into this one.
For myself, I'd want to go back to my ancestor who first set foot on the North American continent, Dirck Volckertszen, and visit in a couple of time periods. First, I'd like to visit him before he took the journey (maybe around 1628) and find out what motivated him to go to the new world, who his parents and grandparents were, brothers and sisters, and find out all about his past.
Then, I'd want to skip ahead through the journey to when he set foot in New Amsterdam around 1632 and watch a few moments in his life that were recorded - rumor has it he was a hard drinking man, a bit of a gambler, and had a terrible temper. He paid his debts, both good and bad. Took a fence post to an unruly landlord who dragged his wife out of their rented house by her hair. I wonder if I might like the guy.
And for my wife, I'd want to go back, just a few years, to visit one specific point in her past. She has a great-grandfather that so far has eluded me, so I'd go back to the 1910-1916 period in Sussex County, Delaware, and find out who her grandmother and her great-aunt's father was. And I suppose it's possible that it's two different men, so I'd want to know that too.