I did Karate for several years until I moved and couldn't find my style taught in my new location.

It's a great workout, but more than that, it's actually *engaging*, unlike endless hours on the treadmill or pounding pavement, both of which I find incredibly tedious (despite being a runner in high school).

That said, weight gain has a lot to do with diet as well. If you're curious about the biology of nutrition and how your body reacts to different foods, I'd highly recommend Taubes' "Good Calories, Bad Calories". It's a dense but very interesting read.