Lunge Alternatives?


I've been lurking on the message board for a while, but since all of you are so sweet and helpful, I thought I'd jump in with my first question:

Do you have any suggestions for alternatives or substitutes for lunges? I've never been very good at them (my balance throws off my form) and lately I've found that they make my knees hurt. I'd been doing lunges anyway so I could work on my balance, but I think for now it's time to find an alternative so I don't hurt myself.

Is there another move that would work similar muscles (all I can find is a step up, and I don't have a step) or would you just do a different move altogether (like a squat)?