Quite some time ago I was in a car accident where my right knee slammed into the dash. This caused me some issues that still continue to this day. In the beginning walking was a struggle and kneeling was not going to happen if I ever expected to get up again. As time went on and I spent hours a day resting and icing my knee things improved. However, due to this type of frontal knee injury the tendon, PCL, never quite heals. So, I still experience stiffness, strange weak feeling at times and pain if my knee is in the same position for too long. I also no longer feel comfortable sitting cross legged. In the meantime the other knee has developed a clicking sound while walking up and down steps. There is pain every now and then also. Things could be worse and I am not complaining. I am simply explaining why you will find me talking about bad knees or ways to work out with bad knees. I am no expert and nor do I think you should partake in any exercise based on what I say. Please discuss all things with your doctor as knees are nothing to mess with. I am simply sharing information that I have found or what works for me.
Exercises considered safe for bad knees (again consult a doctor as your individual situation may differ):
Elliptical: I love this machine and I am thrilled I can still use it. The Elliptical and give you a great workout without being hard on joints.
Stationary Bike or Cycling: I have seen a trend of old runners turning into cyclists. Why? Knees. Biking is easy on the joints and will provide a great workout.
Swimming: Swimming is great for those with joint issues. You can burn a decent amount of calories without stain on the joints.