...is the title of one of those rather silly Likeness quizzes available on Facebook. As usual, few of the ten answers fit what I would do, and for years I've pondered off and on about writing a science fiction novel around what I'd do, and a friend and I have had several discussions of it.
What I'd do seems to show my librarian's heart... I'd go back to the burning(s) of the Library of Alexandria, and rescue all the materials stored there. Then I'd go find other such lost treasures. For example, the ancient Irish used runes on sticks to record their law codes and more, and I'd love to save them. Another thing I'd do is get DNA samples from great leaders (using, of course, my own definition of great leaders) and study them, and maybe clone them. Would need to think a lot about the ethical implications of that last one.