Bicycle. Go out for a bike ride. It's easier on your knees and builds more strength in your legs. Which will make you faster when you actually do decide to run. For a more intense experience try doing Hills. Aside from the physical benefits, it's just a nice way to tour unknown parts of your neighborhood.
Jump rope. Many of us have not done this since grade school, however it's an incredible workout. It may take some time to get back the coordination but it's worth it. After just two minutes of jumping rope most people are in their aerobic zone, breathing hard and building endurance. Keep it up for a bit longer and you'll start to build muscle endurance in your shoulders from holding the rope up for an extended period of time. No wonder professional boxers and martial artist use this as their go-to exercise.
Airdyne bike. This is an often overlooked and underappreciated tool. The Airdyne bike allows you to use your arms as well as your legs to propel the wheel. The wheel is a fan that uses air resistance to match your intensity level so the harder you pedal the more resistance is produced. What makes this tool so valuable is that you can just use your arms. So now on those days when your legs are too sore to run, bike, or jump rope you have an option to do cardio just using your arms.
Hopefully this list of non-running workouts helps you to mix things up!
Scott Bitterman, CSCS