bike! regular biking is a great way to a fantastic ass. I swear, I grope my own ass.

I've also heard that stairmaster is bad for the knees. Perhaps lunges instead? I do them with weights in my hands held at my shoulders to activate my core muscles. I don't even do full steps: I simple stand with one foot out front and one out back and do reps of lowering one knee to the floor and flexing the hip and knee of the forward leg.