First everthing must be done standing, it is a standing sport. This alone will challenge the core. Make sure that the inntrinsic core is firing with the extrinsic core and there are no compensation patterns. If all that is fine then the next level is doing everything in the workout on a single leg then progress that through different modalities of progression. Such as, Airex pad, half foam rolloer, dyna disk, bosu, and voodoo board. This will train stabilty integrated with movement patterns. If extra core work is required then go with planks all around, some Gray Cook chop and lift patterns, and good ol deadlifts for posterior chain strength.
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.
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.
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.