I recommend kitesurfing too. ( But take lessons to start with. Dont try it on your own. I've written some articles on kitesurfing for beginners that you might find useful - especially the safety aspects: newkiter.com) Kiting is dangerous so please get lessons from a good school.

But yeah I think anything that gets you to physically exert effort, allows you to feel some sort of accomplishment and which allows you to progress slowly would be great. Even the gardening idea given by another poster is a good one.