Leg Extensions from Ball
This exercise will improve balance as well as your core, quadriceps, and hamstring strength.
Sit on a ball, tall and upright, with your feet no more than shoulder-width apart and your hands resting on the sides of the ball. Tighten your abdominal muscles. Keeping one foot flat on the floor, extend the the other leg fully, tightening your thigh muscles. Hold for 10 seconds. Repeat 5 times with each leg. If your hamstrings are tight, you will not be able to fully straighten your leg, but go to where it is comfortable and hold that position, trying to improve each time.
Remove your hands from the ball and keep them out to the sides so your body forms a T. For even more of a challenge, add ankle weights, starting with 1 or 2 pounds. Also, try doing this while on the ball of your foot, rather than keeping your foot flat on the floor.