Tai Chi is probably one of the best forms of low impact exercise. It develops muscle and balance and believe it or not is also a form of martial art if practiced correctly. I took it for a couple of years and it really exercises muscles you don't normally use. If you practice it, do it in the martial art form and not the dance form. Ever do a kick or a partial one leg squat in extreme slow motion? Try that with the 108 moves...you'll have long lean muscles in no time. One of the things that Tai Chi lacks is it doesn't exercise your mid section and core too well. You'll have to stick to traditional calisthenics for that.
"You haven't lived until you've died." - various