Let's say it's a fairly big one - 20 million or more immediate payout.
1. Quit your job? Volunteer?
2. Take more risks in your career?
3. Live on interest only?
4. Donate a lot to charity/church?
5. Buy a huge house? A beach house? A mountain house?
6. Travel lots?
7. Go MC Hammer on us?
I'd probably keep my job for a year or two minimum just so my life wouldn't change too fast. It would be easy kind of going through it with an I don't care attitude. I would try to stay as down to earth as possible. I would get a nice house but not a lot more. I would just enjoy the freedom and travel a lot.
I rarely pay the "tax for the mathematically challenged" but we'll occasionally do it just for fun.