I used to have a flat butt. When I moved to New York, I became exposed to stairs. After several months of climbing, I noticed a bubble form back there. And boy is it firm! So I suggest stair steppers or, if you have access to a stadium(football, baseball, etc..) Climb Bleachers.
You can definitely do soe squats. They work out great.!!! Even wall balls. You take a medicin ball, preferably a 14 pounder, then you simply do squats all the way down and jump up when ou throw the ball up and against the wall.be sure to catch the ball with your hands likea Y. Thats how ou throw it too. Its simple really bt if I confused you which i prbably did then just look it up. it tones your arms and woks your butt