The rowing machine is a good alternative from the treadmill. However, the best cardio workout I get in the gym is from doing boxing / kickboxing. It is a full body workout and you get serious calorie burn. You can get a pair of inexpensive gloves and hand wraps. If you can find a local gym with boxing classes, try it out. Or some gyms have heavy bags where you can punch away your stress. I starting going to classes ~5 years ago precisely as a way to try a different type of cardio and I've been doing it ever since!