I'm looking for some good types of cardio exercises that are good for someone with knee problems, especially knees that have patella dislocation problems. I've tried the elliptical machine and found it to be far too boring for me to want to use on a regular basis. I also used to surf a lot, but that's really painful for my knees. So, I need something new to try that won't be boring and won't cause me excess knee pain.
Any type of low-impact cardio, but swimming would be the best since there is zero-impact. Stay away from any type of jumping or rotating activities as jumping notoriously leads to dislocated patellas.
Other than exercises recommended by @Alison S answer, you can also try upper body cardio exercises, such as rowing (or rowing machine) or kranking.
Plus they will also improve your upper body.