If it were my last day (as in the world wasn't going to end - only me) then I would try to initiate as many "pay it forward" ripples as I possibly could. The closest thing we can get to the purpose of life is establishing a world that became better due to your existence. Sounds idealistic, I know, but by causing a positive ripple I think it's pretty possible to achieve.

As for personal indulgences, I'd eat a ton of tacos, get drunk, and listen to Tool all day.