Lunges

The lunge stresses the quads, glutes and upper hamstrings without the heavier weight of the squat.  Stand with your feet shoulder width apart.  If you desire to have additional resistance, you can use dumbbells or a weighted barbell behind your shoulders.  Step forward with one foot and bend the knee while stretching the other leg backward so that it nearly touches the ground.  The front knee should not overextend the foot as this can place undue stress on the knees.  Return to the original stance and repeat with the other leg.  Alternatively, one can performing walking lunges if sufficient space is available.  Some gyms have a "lunge lane" specifically for this purpose.