Aside from saving your knees, the low-impact elliptical can be a time-saver too, mixing a little strength training into your cardio. Hopping on a machine with moving handles means you can work your upper body; push the handles to work your chest more, and pull the handles with focus more on your back. Skipping the handles altogether gives you a chance to focus on your core. Just place your hands on your hips and keep the movement as smooth and steady as possible. And pedaling in both directions means you work both the quads and the hamstrings nicely.
Here's a half-hour workout that does all that, while getting your heart rate up too! Want to check it out?