BRING SUNSCREEN! Even if you have a tent to sit under. The sun will slide off to one side and you will cook if you're not careful. And bring a hat with a brim. If you share the sunscreen you will make great friends with your neighbors!
Also... I would strongly suggest that you keep your money in a waist pack, not a cash box. I learned that the hard way, at what *should* have been a very profitable Thanksgiving weekend craft show long ago.
What is it that you make?