When I first began rehab I was told that the one legged squat is the most useful exercise for recovery. That has certainly been the case and I have done a lot of one legged squats in all sorts of different formats. An easily achievable one is shown below. I can do three sets of squats where each set consists of 10 to 12 repetitions.
It is important to try and touch the nonweight bearing knee almost to the floor in the squat before driving straight up with the weight bearing leg. Back must stay straight.