Deadlifts are an excellent exercise to strengthen your entire backside (glutes, hamstrings, back) while also engaging your core, without putting too much pressure on your knees and hips.
How to do it: Grab a pair of dumbbells and stand with your feet hip-width apart, palms facing your thighs. Push your hips back behind you as you slowly bend forward from your hips, stopping when your torso is parallel to the ground (or when you feel a stretch in your hamstrings). Return to standing, keeping back straight and abs tight. Try up to 3 sets of 15 reps.