The "Fire Hydrant" exercise strengthens your legs and hips.
Start by getting down on your hands and knees. If it feels better, you can use a soft mat under your knees for comfort. Lift one knee up and move it to the side, kind of like a dog lifting its leg when it's near a fire hydrant. Hold it up for two seconds, then gently bring it back down. Perform 8-12 times with one leg, and then switch to the other leg.
Progression:
To make it more challenging, put a water bottle next to you. Lift your knee up and move it over and around the bottle, going forward and then backward to where you started. Keep doing this a few times, and you'll feel your muscles working hard.