Going for things that aren't likely to happen unless you set out to do them. There's plenty of must do stuff that you will do anyway

1. Watch a total solar eclipse
2. Look at the rings of Saturn, live through an actual telescope, not through Discovery channel
3. Go to a really really dark wilderness area on a clear night and look at the stars the way everyone saw them before coal and electricity
4. See at least one truly wild and dangerous animal (lions, tigers, grizzlies, whales, sharks...) close up, in the wild
5. Own a pet
6. Snorkel and look at fish underwater, in their natural environment
7. Go look at the giant sequoias
8. Climb beyond 12,000 feet
9. Make an enemy
10. Walk away from a tempting opportunity
11. Voluntarily choose less money
12. Live in a foreign country
13. Read a book written more than 2000 years ago that is not a religious text
14. Smash something in anger
15. Be in a life-threatening situation
16. Fast for a day
17. Get punched
18. Punch someone
19. Run away from a fight
20. Make a complete ass of yourself in public

Hmm... interesting. That was a quick brain dump. And almost everything on my list has to do with nature, with the rest having to do with being a sociopath/antisocial.

I've done all of these incidentally, except 16, which I plan to do soon. Will look for ideas in others' answers.