How do i circuit train with a bad knee?

Avoid: squats, sleds, leg extensions, treadmill and elliptical. Consult your PT. Upper body work only and consult an orthopedic expert with a background in sports medicine. No running, jogging or biking-clear?

Low impact.: You would want to focus on lower impact exercises. Running/jogging may be out of the question. Try walking, swimming, exercise bike, elliptical. Hiking may be tough depending on the terrain and slopes you try to conquer. Weights are fine, but maybe not heavy heavy squats or leg presses - keep weight moderate and enact slow controlled movements.