Depends on how exactly you want to get in shape. If you want to get stronger and build some muscle, then as suggested above compound exercises are the way to go.
If you want to lose a little weight and/or improve endurance then there's a tonne of cardio options out there. I agree with the plyometrics suggestion. Its old fashioned, sure.. but also an incredible workout.
I'd recommend Yoga even once a week. It helps tone your muscles, improve core and flexibility amongst other benefits.
Just as important is your diet though. Make sure you eat the right foods to give you the energy for training and snowboarding days.
Always allow some recovery time between exercising and snowboarding. I've been guilty in the past of goin' boarding the day after a heavy gym session and it proved to be a killer on my legs. Denying yourself sufficient recovery can be detrimental to your fitness so, you have been warned.