The best exercise for snowboarding is one that strengthens the legs and core muscles. This will help to improve your balance and stability while riding.
Squats, lunges and leg lifts are all good exercises to help build strength in these areas. Pilates and yoga are also great for improving flexibility and strengthening the core muscles.
If you are unsure about what exercises to do, it is always best to consult with a qualified personal trainer or your doctor before beginning any new workout routine.