Squats work just about every muscle below the belt, and they also keep your abs and back engaged too.
To do it: Stand with feet slightly wider than hip width, arms down by sides, holding dumbbells [as shown in photo A]. Bend your knees, hinge at the hips, and push your hips behind you (as if about to sit down into a chair), reaching arms down by sides of legs [as shown in photo B]. Hold for one count. Push down through both feet evenly, squeezing your glutes together as you stand back up. Do 3 sets of 12 repetitions, taking a short 45-60 second rest in between sets.
Form Tip: Watch that your knees track over, but never past, your toes as they bend. Try lowering until your thighs are parallel to the floor, or as low as possible, without losing your form.