Running is a great form of cardio, but not if you have bad knees. Give one of these other great exercises a try instead!
Elliptical
Great for low- to no-impact lower body exercise.
biking
A recumbent bike is less stressful on the knees.
water aerobics
Swimming or pool exercises are very gentle on all joints.
circuit training
Circuit training with weights can increase the heart rate as much as a cardio workout!
boxing
Jab, cross, and kick up a sweat! Air punching is great cardio if you have lower body injuries.
walk
Get health benefits with low-impact walking. Aim for 30 minutes, 5 days/week.
battle ropes
Ropes can give you a full body workout without ever moving your feet.