Skating treadmills, or skate mills, are in my opinion the best way to train your cardiovascular systems in the off season. The biggest draw back of the skating treadmills are that they cost a lot to use. They have tremendous upside though, not only can you work on your cardio training but at the same time your are strengthening your skating muscles and working on your stick handeling or shooting. There are lots of drills that can be done on the skate mills, all sorts of stick handeling drills, passing, and shooting all while skating at your top speed. This will transfer onto the ice much more than any other off ice training. Agility drills can be done with the skate mill inclined, quick start drills to work on those first few steps. Another helpful drill is to be skating at full speed and have the instructor push you while you try and stay balanced, teaching you to keep your feet moving while there is pressure on you.