'Step-ups' is a gentle exercise once you are mobile to start building up quads strength and a bit of balance. Start stepping up with the bad knee.
If you cannot view the video, here is a still sequence to show this exercise.
|Stand in a relaxed position in front of a low step.|
|Step up with the bad leg.|
|Step up with the good leg, to stand on the steps.|
|Step down again, starting with the bad leg and following with the good leg to stand back on the ground again. Start the regime with 10 reps. If it is too easy you can use higher steps.|