Wall Squat

Find a nice flat piece of wall and stand with your back leaning against the wall. Slowly slide down the wall while moving your feet away from it, until your thighs are parallel to the ground and both your knees and your hips are bent at a 90° angle. Cross your arms in front of your chest and hold this position for 30 seconds.

Variant: put a big inflated rubber ball (like a small basketball) between your knees and squeeze the ball while holding the squat position