Can you stand balanced on one leg? I do a lot of that; like when washing dishes or emptying the dishwaster or ? I stand on one leg as long as I can, then the other. Uses many small muscles in the lower leg for balance, and engages core meaningfully.
When you get good, do it with eyes closed.
And stand on one leg, bend down and pick up something off the floor without putting the raised leg down.
To avoid too much stress to the knee, keep the weight going down through the back of your leg and through the HEEL, so keep positioned as far back as you can and still stay balanced.