A really big one for me is mountain biking. I always found it to be a great escape from whatever I had going on - school, work, relationships - you can really forget about all of that stuff on trails. Mountain biking at times feels more like an addiction, to be honest. Indeed, I own a shirt that reads "Mountain biking is as addictive as crack, but twice as expensive". I start feeling bad about myself and getting a bit anxious if I don't go out on trails for several days in a row. It sure feels great afterwards though - really push yourself, have fun, get that dose of adrenalin and be physically exhausted in the end. I always solve my toughest problems after "hardcore" biking sessions :-)
Having talked to some hospital staff during my several biking-related emergency room visits, I can tell you that some forms of it (downhill-type stuff) are very hard on your joints, so don't over do it. If you're really into downhill racing and freeride, mix in several good XC sessions for cross-training. You'll be stronger and healthier.
P.S.: another thing that really helps is having an active S.O., someone who'll drag you out of your work-mode and take you hiking several times a week, and who will want social interactions outside of "reading HN".