mountain biking. technical descents require the same mental skills as a technical descent on skis. the uphill climbs are great cardio workouts. get out your wallet, swallow hard, buy a nice bike, and you'll never look back. :)
Yeah, I think many of the mountain biking skills can help with skiing and vice versa. Skills such as line choice, balance, and one of the most important-looking ahead! The fitness benefits are a given.
Use inline skates on smooth inclined pavement. Add poles and you are really skiing, no skidding all carved turns. Great for technique, plus it really teaches you to stay out of the backseat because there isn't one. Good exercise as you skate back up the hill for your next run.