Jachut

I would imagine your knees will stop you doing much high impact? Walking is probably what you can handle now, but you could add some push ups and dips and circuit style exercises perhaps? Can you handle cycling - such as a spinning class or a boxing class? Swimming perhaps?

Whatever you do, take it easy on your knees. Anything that gets your heart rate up, you can keep up for half an hour or more, and doesnt hurt will work.