Cycling
If you like to cycle, you can use either an exercise bike or ride your bike outside. Cycling can be great as it can help you burn off a lot of calories and help you keep your knees safe. Just ensure that you wear a helmet when you cycle on the road because that can be very risky if you hit or fall and hit your head.
Try to ride on flat surfaces and stay away from the hills because hills can cause problems for your knees. Keep your cycle seat high up, too, as this will keep the pressure away from your knees. Your legs will inevitably have to bend while you cycle, so if your pain does not reduce, then keep away from the bike.
Step-Ups
You can use an aerobic bench and do step-ups as a form of cardio. This will keep your knees safe from any impact as you can try and step up well and try not to cause any sudden movements to keep your knees safe.
Pilates
When people consider exercises for bad knees, pilates does not come on that list. However, pilates is an essential exercise, especially if you have knee problems. When you do pilates the correct way, your posture improves, you get better mobility in your joints, your muscles get more toned. Since you have issues with your knees, try and avoid exercises that involve balancing one leg, as that can cause your knee issues to flare up.
Conclusion
If you have been having knee pain for a very long time, it is best to do some of these exercises as a replacement for other more impactful activities. If you never had any knee problems, but it just started more recently, then it could be that you might be over-exerting yourself in some way.
Try and stay away from running on hard surfaces etc., and most likely, the pain you are experiencing should go away soon. If the pain persists, you might have to see a doctor, but in the meantime, you do not have to stop exercising, and it would be best if you took up one of the exercises above.