Here is an exercise I've been working on that has been challenging and fun at the same time. It utilizes the TRX and the Indo Board Roller to do a 1 Leg Scorpion and atomic pushup. I usually do about 8-10 reps of each while trying to maintain my balance. This is great for SUP! It is so core intensive that the exercise is fantastic for training your balance, stability and strength - all needed for stand up paddle surfing.
It's difficult! Let me know how it goes!