I guess the idea is to go ahead and live like you only have six months, just in case you really do only have six months (or less). But one thing I would do is write long letters to all the people I love to make sure I'd say the things that I really want to say. I'd also write that book I've been putting off, although I'd be hard pressed to get it together in six months.
I expect we'd all like to think that we'd leave something behind that made our life worth living. Whatever that is (or at least whatever we think that is) is what we'd do.