You might want to look at the Leslie Sansone- Walk Away the Pounds (WATP) series too. I have an extremely screwed up knee (use a cane when I'm out of the house most of the time, kneecap doesn't track correctly even after tons of surgeries...it doesn't help that I tore one of the muscles after a surgery as a teen that wasn't discovered/diagnosed/treated until I was almost 30). I can do the WATP- there is no fancy foot work, just walking in place with occasional gentle side steps, light kicks, & knee raises. The other good thing is that you are 'walking' on a flat, even surface in your own house- I know for me, walking on uneven surfaces is still one of the hardest things for my knee, and when I'm most like to have problems with falls due to my kneecap dislocating. I'm with you on not being able most of the aerobics type tapes- I end up concentrating so much on what my feet & legs are doign that I can't keep up or just end up frustrated...even with the Richard Simmons ones.
The stability ball exercises are good too. I love to use my stability ball at my computer instead of my chair to work on my core without having to actively workout. They are great for workout...and fun too. You can pick them up at WalMart even. If you're a little unsure of unstability of a ball, you can start with a capsule ball (I think they look like 2 peanuts in the shell
)- they you only roll in one dirrection instead of all which ways.