Hi. I've had knee problems, arthritis and a torn meniscus, so I'm not supposed to be on my feet or do things to strain my knees. Instead of treadmill and bike exercise I swim regularly. Between each lap I do a series of crunches, pulling my knees as close to my chest as possible, and alternating sides. I started with 10 crunches per lap, and now am up to 40. I usually go for a 20 lap workout, which will give me 800 crunches per workout. While I haven't lost much weight yet, i am developing much better abdominal muscle tone. 

I also do a set of weight machine exercises before the swim, which include a crunch machine, which seems to work the muscles differently. I'm doing 3 sets of 20 crunches there at 110 lbs.

I feel that the swim crunches are good for the knees, since each crunch goes from complete extension to the highest I can go. It gives maximum flexing of the knee joints while having no weight pressure.