Hey docs, what hobby could an nonathletic 13 year old do to stay fit?

Even just walking: Do you like animals? Volunteering at a local shelter where you can run around with or walk dogs would be a good activity. Take up biking. Play Wii games like tennis, even bowling. And then there is plain old walking. You can do this by yourself, or go to a local nursing home and take residents for walks in their wheelchairs. They have great stories about history for you to learn while you walk!