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!