Always make sure to bring a water bowl and more water than you think you may need. Water gets spilled, you may share with other thirsty dogs, or you may need to pour it on your dog to cool their body.
If you are going to stay in one spot, then bring an umbrella or a tent to provide shade for your pup.
If you will be on the move, make sure to bring a covered wagon that your dog can comfortably lay down in while you are walking around.
Freeze some water bottles so the dog can lay down next to them and it will help keep their body temperature down.
Stay off the concrete especially the blacktop. Dogs let out heat mostly through panting but they also have sweat glands on their paw pads. If the dog is walking on a super hot surface, then they can't release the heat through their paws.
Put cold, wet, rolled up hand towels in the cooler so you can pet the dog with this cooling treatment.