For a low impact exercise I would suggest walking for atleast 30-45 minutes on different kinds of terrain. You can walk uphill and downhill or even use a lot of stairs as well as on regular flat terrain like walking in large malls. Doing this would tone up your leg, calves, thigh muscles and it would also help you develop your abs and other lower body muscles. It's easy to do and you can do it both indoors and outdoors or a combination of both.