You're talking about doing squats against the wall with a stability ball. Yeah, those are a killer.
One core exercise, on your back, your shoulders and arms touching the floor, your knees bent so your feet are flat on the floor just a little wider than your shoulders. Concentrate on engaging the abdominals and then bring your hips and pelvis, by contracting your glutes, up off the floor while keeping your shoulders and arms in contact with the floor, and hold for 30 seconds to a minute at first.
The kickback machine (can't recall the exact name), by resting your heel on the footrest, facing forward and standing upright, kick back your leg, concentrating on your glutes.
If anyone has those treadclimbers that looks like a section of an escalator, doing that with correct form really hits your glutes, but when cooling down at a slower pace, incorporate the kickback motion, and damn it burns.
Squats or front squats too. I'm assuming everyone knows about squats, but front squats are a little different. Feet close together, just barely touching, and the bar is resting in front of you on your shoulders but not touching your neck (you do need to breathe, you know), held in place with your arms crossed, your hands balled into fists. Use a plate or plank to stand on so your heels are resting on it, but your toes are touching the floor. Keep your back relaxed and straight, your abdominals engaged, head looking forward, just like a regular squat, and go down until your knees are at a 90-degree flexion, then push back up.
Hack squats are an option, as well as lunges. Just want to make sure when doing lunges that your knee when at the point of flexion does not go past your foot, that your knee, ankle and heel are kept in a straight vertical line. Puts too much stress on your knee and ankle. Hurts like the dickens.