trying to imitate skiing exactly is not nearly the best way to train in the off season, its much better to get into something else for a while. youll find you have much more control/awareness.
but for just physical training, road biking really cant be beat, specially for building quad strength and endurance. weight lifting is a good bet too, but what you really want to work on is stability training and balance. Its much more important for skiing than useless bulk. trail running/hiking is great, do lots of stuff on one leg, like squats and stuff. it makes your knees a billion times stronger, and helps tremendously with your on snow coordination and injury prevention. yoga/stretch sessions to keep limber.
as for actual other sports and such, mountain biking is a great cross training tool, its way more of a whole body thing than riding road, and alot more fun. inline can def help a bunch on rails, and trampolines are prime for keeping your air sense up. but really, like i said, anything that keeps you active is good.