I want to tone up my thighs, but I have bad knees (patella tendonitis) so doing squats hurts. Holding my weight on bent knees is impossible sometimes. The more I play sport the worse they become.
Does anyone have anything low impact I can do to tone my thighs.

I'm not sure how these exercises will effect your knee, but some ideas:

Walking lunges (for more resistance, add weight OR do lunges up hill)
Jumping lunges
Box jumps
One leg step ups
Timed stationary wall squats (while squeezing medicine ball with knees)
TRX lunges
TRX one leg squats
Hill Sprints
Tire Flips
Tire Hops
Ladder drills

If you need to see exercises, let me know and I'll post a vid.

God, I have *GOOD* knees (knock on wood) and even I cringe from one leg exercises. 

Honestly... anyone else like this?  Ha, *ZERO* interest for me in one leg squats, lunges, etc.!

I've played competitive Tennis since I was 12; as a result my workouts aim to improve not just strength, but balance, agility, flexibility (which is where I typically fail), and explosiveness.  I've gotten to the point where I can stand on a fully inflated swedish ball and stay balanced while doing squats.  I believe my joints (knees, ankles, hips) have become significantly stronger as a result.

For what it's worth, none of the exercises mentioned in my previous post are that complex/advanced.  The TRX one leg squats and the TRX lunges are the only exercises I would recommend instruction before doing by one's self.  Walking lunges?  Box jumps?  Sprints?  These are staples in any lower body/body weight/interval workout.