Wanted: Quad excercise that does not involve the hip flexors.

So here's my problem: Due to weak glutes and core, I have a very overactive and tight hip flexor on one side. I understand that I have to strengthen those weak muscles in order to regain normal function in the hip and to stop my hip flexors to engage all the time.

WHile I do that, I would like to still be able to do a little work on my quads. What excercises can I do? Squats and Leg Presses are out of the picture, since those heavily engage the hip flexors in somebody with my condition.

Any ideas?