When you're looking for a break from your usual treadmill routine, you might want to try an elliptical machine. They have some nice features: Most of them have a lot of variables built in, as well as movable handles you can use to work your upper body. Elliptical machines are especially good for people with knee problems, who find the workout to be a great cardio alternative to running because it's much easier on the joints.
Some elliptical trainers have adjustable ramps, which allow you to target different leg muscles by varying the incline. If the ramp is inclined and you're pedaling forward, you're working the muscles on the backs of your legs: the hamstrings, glutes, and calves. If the ramp is at a lower incline and you pedal backward, you're training the muscles in front: the quads and dorsiflexors.
The one thing that makes me wary about elliptical trainers is that it's fairly easy to phone in your workout — especially if you hold on to the static handles. The only handles you should be holding on to are the movable ones that get your arms and heart rate pumping. It's really important that you keep your intensity high. And don't trust the machine to tell you how many calories you're burning — those electronic readouts are not the most reliable. Instead, monitor your heart rate and keep it at 85 percent of its maximum (to calculate maximum heart rate, subtract your age from 220). Increase the resistance so that gravity isn't doing all the work for you. Remember, your time is precious and your workout is for you alone — don't cheat yourself!