Whatever you find. Most comfortable and enjoyable with cross training recommended to avoid overuse injuries. Swimming, biking, walking, jogging along with the use of various gym equipment except possibly a rowing machine. In addition, stretching routinely and core strengthening exercises should be part of your routine.
Low impact. In general I recommend low impact activities such as bicycling or an exercise bike, an elliptical machine, or stairmaster. Swimming is also great. Remember to also do a lot of stretching to keep your spine loose, yoga and Pilates are great for this.