If today was my last day, well, I'd probably go through the 5 stages of grief, but if I overcame that, well...
I'd try to think of something I could do that's amazing, but doesn't take a long time, hmm...
HMMMMM this is a difficult question.
I'd probably document my thoughts and then do something insane without consequence like skydive or base jump or something. Just to experience something that not many people have.