1. Parkour.
2. Karoke in public.
3. Breaking a plank with head or hand.
4. Mud wrestling (intentional).
5. Mosh pit crowd surfing.
6. Join the circus.
7. Join an ashram.
8. Drinking champagne from a shoe.
9. Games (scrabble, WWF, crosswords) champion.
10. Collecting tchotkes--shells, cat figurines, owl related items, etc.
11. Doing anything in high heels, least of all dancing.
12. Wet T-shirt contest.
13. Competitive about the number of visas/stamps in my passport.
14. Excited over the number of digits in my paycheck/tax refund.
15. Jazzed about those capital letters after my name.
16. Counting number of Friends on FB.
17. Single-spacing the Christmas/Hanukkah letter.
18. Giving up on life.

(ideas in part ganked from AARP article)

I love emjay's additional list in comments--be sure to read it!