If you don't want to put a load on your knee but work the thighs then get a football. Pump it up, lie on your back with the ball between your thighs and try and burst the ball. if you hold your legs up straigh off the ground you will work the abs too.
Also, no I not good at explaining this so bare with me
. Go on all 4s, lift your leg as if you are a dog doing a pee, do it to the count of 4 or slower, and do as many reps on each leg as you can. also doggy style, push one leg behing you straight about an inch off the groundthen bend the knee to work the hammie, straighten leg and lower.
Lie on your side top leg is out at 90o from your hip, knee bent, lift the back leg (which is straight) about an inch off the ground, then lower. Do them slow and controlled.
The knee is the weak joint and if you can build the muscle around it. Yoga and pilates are a good send for the hips, core, knees and ankles. I used to have hellish problems after fights with my left ankle, not had one problem since taking up yoga and pilates. Also rosehip, tesco do it cheap as chips, is wonderfull for the joints.