If you want your horse to absolutely ADORE showing, coat some of the treats in hardening frosting!!! Then when you go to shows your horse knows the difference between being good at home, vs the extra sweet reward of being good at shows ;D Hahahaha don't ask why this works... it just does.
I have to agree on ipod/ihome/music of some sort. Its always fun to chill or dance around in down time.
For our shows (idk how long you're going to be there) we bring Gatorade powder. Our horses ADORE Gatorade, the blue kind. 2 scoops to a small water bucket, 3-4 scoops for a large one. We give it to them after a hard work and it cools them down from the inside out, plus they LOVE it!
Lastly, don't forget to bring a good phone or camera, and an extra person. Someone has to take pictures of you and all those ribbons!