Back squats and dead-lifts are outstanding for leg and core strength / endurance. Olympic lifting, kettlebells, and plyometric drills are great for developing speed and power. As a few people stated above, yoga is also a great way to tune into your body and build balance and proprioception.
Now here’s my opinion as a long time snowboarder.
There is a tenancy to incorporate “functional” exercises like balance boards in your programming, but remember, the body adapts to exactly how you train it. Getting good at balance boards, well, makes you good at balance boards! Specificity is key. If you want to get better at snowboarding, you have to get out and snowboard!
I would argue for yoga, especially for styles that are principally standing postures (.e.g Bikram-esque). Postures like tree pose, dancers pose, standing forehead to knee, etc. Other benefits:
-core stability improves
-increased flexibility (which could only help should the athlete fall when snowboarding, for example)
-balance out the R and L sides of the body – since snowboarding is an “unequal” sport in terms of which leg is always in front etc.
Look into tight rope walking. May sound crazy but you can start off with a decenntly thick strap and narrowit as you get better at it. Intense core stability and balance workout. Good for snowboarding, skiing, surfing, skating and anything that might require quick postural changes for the sake of balance.
Wobble boards are a good tool to use for snow boarders. Design exercises using this tool that mimic as close as possible the actual sport. Challenge the client’s ability to adjust to varied demands will maintaining balance on the board. Don’t forget to train the legs, strengthening quads, hams, ankles, and calves. Cannot make the moves without your appendages. Strength does enhance balance.