Squats, bulgarian split squat, lunges variations, deadlifts, romanian deadlifts, anti-rotation abs exercise (pallof presses, etc), rotational abs exercises (cable woodchopper), and TRX exercises. Crunches are generally useless. Those will strenghten the main muscles needed for snowboarding. I guarantee you will notice a difference once you start focusing on those main exercises. I never get sore once after snowboarding. For specific balance skill, just go snowboarding, longboarding, or skateboarding. Nothing can best replicate the movements other than by doing the sport itself.