Good thread! For me, in no particular order:
1. Being out on the water - C-Dory, kayak, inflatable, whatever
2. Dogs
3. Watching wild critters and birds
4. Reading - fiction mostly, non-fiction on foreign policy, military, intelligence, history, philosohy, religion
5. Kids - sharing time with them, introducing them to new things, watching them grow
6. Wife - like kids, a lot more than a hobby, but she's more fun, interesting, challenging, and rewarding than anything else on the list!
7. Travel - seeing new places and experiencing the bad and the ugly as well as the good.
8. Public speaking - high school groups, Ph.D's, elderhostels - whatever the audience, I get a kick out of engaging with groups on topics that I (hopefully!) know something about
9. Motorcyles
8. Off-road driving
9. Camping