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.
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.
You might swim with a pull buoy : www.youtube.com%2Fwatch%3Fv%3DITK0chbKegI&usg=AOvVaw16R0fot4UEMgQmyP75U7sr
Otherwise if your patella problem still allow you to walk or run you should definitively try with minimalist shoes (you can see brands like Merell, Vibram for Vivo Barefoot for this). This allows to have a natural biomecanic movement, and thus, not letting all the power of your steps traumatizing your knees.
Indeed, you will have shorter, easier, lighter footsteps. The impact will be absorbed by your calves muscles. Your ankles will be free to play their roles (stabilizing). Your feelings and feeback will be much more pleasant.
But beware ! This is a very different effort and your calves will be very sollicitated. You need to re-learn to run with them. Begin very low and go progressively. If you are interested, I strongly advise you to follow this guide : https://us.vibram.com/on/demandware.static/-/Sites-VibramUS-Library/default/dw5f33655e/natural-training-guide.pdf
In any case, you really should ask to a sport professional to do a checkup and go progressively. Good luck.
Try cycling. I ran for years, on whatever I wanted, including lots of concrete. One day I went from an 8–14 mile run/week to barely being able to walk down stairs thanks to extreme knee pain. I took up cycling 10 years ago and never looked back. I could never get into indoor trainers (though recent digital/social accompaniments look to make it more interesting) opting instead for anything outdoors, especially in nature. I'm talking mountain biking, gravel biking, road biking, and everything in between. I have absolutely no knee pain anymore and can ride huge distances / elevations, maintain conversations with friends, get to work, etc. Highly recommended.