firming up my butt??

I've been going to gym for almost 2 months now, 5 days a week, mostly doing cardio and some free weights. I've lost 8 kilos (17ish pounds) in this time.

However as I lose weight I notice my butt sagging more than I would like. Now I know that I can't help much if I end up with loose skin in that area. But are there any exercises I can do to firm up my butt and hips under neath that skin (and possibly that might help avoid the skin sagging to its worst)?

Just an fyi, I can't do anything even resembling squats or lunges. I have incredibly sore knees after a single set of these exercises, so I have been avoiding any kind of leg exercises (as a result for the first time in my life, I've started to look a little bottom heavy). Any kind of pressure or weight on my knees, as such, is a big no-no.

Can any one help with some ideas? I'm willing to buy equipment (like a fit ball etc) if that will assist at all.

Thanks in advance