Bike riding, and renovating and repairing old bikes (bicycles, not motorbikes.) Going for a 15km bike ride is a great way to clear the cobwebs and think about a problem in my novels - it requires a LOT less concentration than driving.
Woodworking and carpentry - no practical use, except for the thinking time.
Target archery, although it's been a while now. Takes a lot of hours out of the day, detrimental to writing.
PC gaming. Not conducive to writing at all ;-) I do like sandbox games though, where you just build stuff. Minecraft, Space Engineers, that kind of thing. It's creative, not destructive.
Board gaming with a couple of close friends. We play complex games, really good for keeping the brain sharp. I'm no slouch, but the pair of them usually beat me. I don't mind, because it makes it sweeter when I do manage to win at something!
Digital art - just plain fun.
Computer programming is a big one. It's a hobby, mostly, but if I was forced to give up writing or programming, I'd give up writing. (Financial considerations aside. I just love the way programs come together from nothing. There's that constructing/building gene again.)
In the past .. sailing, windsurfing, a few track days with my car, that kind of thing. Bet I've missed a lot out.
If I had the time and money I think I'd go for a private pilot's licence.