i agree, any exercise that you can continue at a nice pace for at least 30 minutes, is good. for most people it is bicycling(of course outside, not in a gym) or running. Also if you do mountain climbing, it is even better, because then you exercise the whole time that you're hiking.
Also i like the following non-cardio exercises for this purpose:
1)hang on a pullup bar and twist your body sideways to the left and to the right, until you can't hang any more. It is possible to do 50+ repetitions.
2) flutter kicks-lie on your back with your hands stretched behind your back and move your legs up and down, bending your knees slightly. goal is to do this nonstop for several minutes.