For the OP, I'll give as much advice as I can. I have brought my last 3 Puppies-In-Training to Disney World over the last 3 years. Each trip was different and we experienced different things. A few things that I have learned though might help you out.
- I carry a backpack that contains everything the puppy is going to need. It has cleaning supplies, a collapsible dish, bottle of water, a serving of food, brochures and cards (I train for an organization), extra poop bags and an updated Health Certificate of her current vaccines. I also carry booties and a container of Musher's Secret for their paws. It's not a huge backpack, and it isn't as heavy as you would think. I also carry a treat bag in case a bribe is needed.
- The ground can get very hot depending on when you will be there. A good test would be to put your hand on the pavement. If it's hot enough that your hand burns, then it's too hot for the pup as well. We have used both the booties and Musher's Secret. Both work well and it's best to use whatever your dog is most comfortable with.
- A cooling vest of some sort is ALWAYS a good thing to have if you go during the hotter months. One that can be easily re-hydrated with water works the best. A darker color dog obviously needs one more than a lighter color dog would.
-I always carry around a bottle of water as a back-up. You might not be near a place that will give you water, so that extra bottle can come in handy. Just keep refilling it at water fountains and you should be fine. During meals, ask for a cup of ice and feed that to your pup. This will help to cool them down and give them something to keep them hydrated.
- If you can, avoid the hottest time of day, the afternoon. Relax in your hotel room or sit by the pool. If you don't want to leave the parks I would suggest going on rides where you are mostly indoors or go do some shopping which will give the pup a time to rest in the air conditioning.
- The potty areas are located on the park map and aren't that hard to locate. Look for a small road sign that has a person with a dog and that's the spot. When I was there last September, the CMs were a little unsure where they were located, so the maps helped us a lot!
- If you are unsure how your pup will respond to loud noises (like fireworks) I would suggest finding a spot that has an easy "exit" so that you can get your pup away from the noise if they start to react negatively towards it. DO NOT keep the puppy there if they are frightened. You will end up doing more damage than good.
The Most Important thing to remember, especially for a puppy in training, is for the handler to know their dog. I know what my pups can handle and what they can't. We will be going again to Disney Sept 21-26 and my pup will be 16 months old. Throughout the summer I will be getting her used to large crowds, loud noises and walking under the hot sun. Hopefully this extra work will help her to become immune to the problems that can be faced at Disney. Work your dog with these distractions over and over and they should be fine. I usually arrive and head straight to the MK as I find Main Street to be the best place to gauge how my puppy will react. If they are fine there, then they will be fine anywhere.
If anyone happens to be in Disney while I'm there, you can find me with the Black Lab puppy wearing a yellow and blue vest.

Any questions, feel free to PM me and I can answer them as best I can. Hope this helped a little!