Every year I have gone, the weather has been different. My suggestion is to take some fans for your stalls... the cheap square floor kind- they are easily suspended where the horses, or you are not tripping over them and they keep the humid air in the stalls moving!
Folding lawn chairs and a big ice chest! We made ourselves confortable in the barn aisles (even took a nap there) and had the ice chest handy with water, sodas, fruit juice, stuff for sandwiches, fruit and snacks that we could eat 'on the run'.
All your grooming equipment- everything you can dream of. And a golf cart, wagon, cart with wheels, something to carry it all on!!! Some of the hikes to and from your truck get overwhelming after a while!
Something to secure your stall doors. Someone's horse on our aisle was VERY smart and figured out how to flip the latch open up on the stall door and let themself out- a few times! It didnt go anywhere, but was found in the aisle several times, visiting with anyone it could and looking for the feed room, LOL
Extra shavings if they are pretty cheap and you have room to take some. We found they cost a bit more there and we like plenty for the stall and the trailer while hauling.
Oh yes, your camera and your check book with credit cards, LOL
If you are staying in your stalls, bring a comfy cot or couch... and some dark sheets or blankets to put up for privacy and to darken it and to not have any flying birdies leave you something while you are sleeping, LOL
HAVE FUN!!! Wish I was going to be there this year.... I am trying for next year if we can get everyone's health issues squared away! It is always fun and exciting- lots of awesome fun folks and stunning horses!! I hope everyone who is going has a great time and a SAFE trip!