It depends. When I'm starved for human contact, relationships and similar subjects will dominate my mind. Even when not starved for human contact, I tend to wonder if I'll ever meet someone who I could tolerate for the rest of my life (and who could tolerate me...).
Otherwise, I tend to think about the future of mankind. Where will we be in a thousand years? A million? Will we destroy ourselves in 50 more years or will we colonize the galaxy? When our sun craps out, will we be in a position to deal with the situation or will we be gone forever? Are there alien cultures out there? If I have myself frozen, could I one day fly around in my very own spaceship?
Edit: Of course, at any given time, I'm just as likely to be (over)analyzing my obsession of the moment as thinking about the above...