The elliptical machine is nearly no impact compared to the treadmill. Obviously, it works your legs. But if you hold on to the handles, you can pump your arms at the same time.
Rowing is also low impact. It works your entire back and biceps. Real rowing on the river is the same, if that's an option for you.
Try Hot Yoga. The room is heated to warm up the muscles and stimulate sweat. When the muscles are warm, the body can sink deeper into the stretches, reducing the risk of injury.
Tai Chi, unlike Kung-Fu focuses on slow and controlled movements. Rarely will there be any sparring in a Tai Chi class, so you'll be safe.