Hey Frank, if you could live forever, what would you do with the rest of your life?
I guess it depends on what kind of immortality we're talking about.
If the answer is "gain mastery over the physical universe and the fabric of reality" (my immediate thought), then frankly, I am far less interested in physics and the universe than I am in literature and people. Even when I'm feeling very cynical about what is and isn't possible, I never want to cross off "personal relationships that last thousands of years, or at least centuries."
If instead I want an answer that says "I will be with my boyfriend forever and that's enough for me" then the answer is what it is.
To answer the question literally, though, I guess I'd write a lot of books and become a cryptozoologist. Seriously. If it were an easy thing to do I'd love to write a bunch of stuff and then just go around personally fighting monsters with really big swords. I don't want to "rule the world" or make people worship me or anything, just live my life and accomplish some stuff that people remember for centuries.