Alternative Exercises

I've been doing an ice cream fitness program for beginners the past couple weeks. Part of the workouts are back extentions done on some sort of platform/bench device as well as cable crunches.

I'm pretty intimidated by the freeweight stuff anyway, but I also have dislocating knee caps. I got brave today and did freeweight squats instead of using the squat machine. I had every intention of trying the back extentions on that bench, but after watching a man use it while I was doing a different lift, I really think its just going to put too much torque on my knees for me to really feel comfortable with trying it out. I looked around for a cable that I could use for cable crunches, but all the ones I see are pretty high up, they're not near the ground where you could be sitting on your knees, holding the cable by your shoulders and doing crunches. I may have seen one as I was about to leave, but I'm not sure. In any case, I have some minor qualms about getting down on my knees on the floor - I can get on the floor without a problem, its the putting my weight on my knees while I do crunches I['m not sure about.

In the meantime, I've done the crunches and the back extentions on the other side of the room where they have the machines. I know the freeweights are better than the machines, and I feel like I'm never going to have the body I want if I can't exercise these areas of my body the way I want to because of my knees.

Is there an alternative freeweight exercise I can do for the cable crunches and back extentions on that bench that will utilize the same muscles and won't be as hard on my knees? I don't mind doing crunches or back extentions, I just need a different way to do them with freeweights that won't put a strain on my knees.