Exercising. I run 3x/week now, and I sleep better, I feel better, I'm thinner, and I snagged a wife. Should have started running back in high school instead of spending my days playing MUDs. (Not really. I don't regret anything... but I could have made more time for exercising.)
Also, playing guitar. I'm hearing impaired, and my audiologist told my parents I could never be in the band at school, so they never pressed me to take guitar or piano lessons. I wish I had learned piano as a kid, because I love music so, so much (despite my hearing loss). That said, playing guitar is one of my favorite things ever. It's so cathartic. I'm really bad at it, but just playing helps melt away the stress of my life.
Also, when I was in college, I dated this gym rat for awhile. She told me one day, "On days when I don't feel like going to the gym, I tell myself to do it for five minutes. If I do it for five minutes and still am not feeling it, then I give myself permission to leave." Then a few seconds later, she said, "I never leave after five minutes."
I've adopted that outlook for almost everything. I don't want to go for a run? I'll run for five minutes. If I still don't want to run, I can stop. I never stop and always run the whole route. If I'd rather stay home and watch TV than accept an invitation to happy hour with coworkers, I'll do happy hour for five minutes. If I don't like it, I can make up an excuse and leave. I usually wind up staying the whole time. This outlook has gotten me to do dozens of things I'd never do: get certified as a scuba diver, take a trip to Peru, run a half marathon, audition for a play, and so on. It's a game-changer, for me.