I'm a fan of body weight only squats. I emphasize squatting as low as you can while maintaining a good straight spine and heels have to stay on the ground. Find something that is the right height and each time you squat you touch your butt to the object (I'll often use the bottom step of my staircase or a medicine ball). When coming back up focus on pushing down through your heels, this keeps your weight back and gets you into the glute max more.

I also really like lunges, especially walking lunges.