Here is a fantastic balance, core, and strength exercise that is PERFECT for SUP!!!  It is a series of 30 partial squats on the Indo Board Roller.  Be certain, just like when on your SUP, to keep your knees, feet and ankles facing forward, go all the way down on your squats, and half way up.  I like to use the bodybar since it adds a little weight and helps keep your posture in alignment.