After the usual play games and watch movies and destroy random stuff until I get bored, I'd learn how to mould metal, and make a bunch of tablets depicting all of humanity's greatest mistakes, for when the next species comes along and inherits the earth. I'd then make a bunch of copies leave as many as I can in places I knew people (Or whatever) could find them.
Because if we're all going to die, with the planet the way it was, the least we can do is set a good example for the next guys to use it.
PS, If the oppourunity came along (Like if I found a secret cloning lab or something) I'd do what Carbonyl did and take a crack at repopulating the human race. Sure, there would probably be some deformaties along the line, (Tails, extra limbs, eyes, ect) because the entire species would be descended from one guy and whatever he could find in the in vitro fertilization clinic, but I think I'd be able to live with that because I always thought the human race could do with a little reboot.
But I'd still put those metal tablets around anyway, just in case my clones mess the world up again.