For my boy:
Lower your expectations on what you will be doing (riding every ride, seeing all the characters, staying out all day, and so on). That way, every little thing you actually get to do will be a thrill.
Prep rides by viewing
youtube stuff. The last thing you want to do is be in the nice cool stretching room at Haunted Mansion only to have you toddler scream bloody murder and have to leave. On the same note, identify places (ride/restaurants) that ARE cool for afternoon breaks
I'm not a RD person, but either do RD when it's cooler outside, or go at night/evening when it's cooler and risk throwing off sleep schedule for coolness sake. You will have to put him back to schedule when you get home anyway.
Hydrate and take lots of pee/diaper breaks. I like/don't like cloth diapers. Reason why I like them -- they prevent wet plastic butt rashes and save the planet. Reason I don't like them - you have to wash or at least rinse them at the hotel. It's easy enough to do, but its a total PITA to do it. Again, pick your poison, but remember to bring diaper rash cream, change diaper frequently, and take care of your toddler's butt. Diaper rash is no joke!
Lather sunscreen on you LO. The sun down here is no joke, and your LO needs a stroller with a LOT of shade (not the crappy shades that cheaper strollers have.) Also a linen blanket to hang over the stroller if needed to for extra shade, and a plastic rain shade. It has been raining every day today - hardcore thunder/lightning rain.
Get a hat for munchkin. With a brim. Get sunglasses. Practice wearing them, it took a while for our son to understand how to use them.
Bring a rash guard for swimming. Sunscreen them up and take breaks from swimming b/c the sun can burn you and you don't even know it when you are swimming.
Stay in or at least be aware that the hottest times are between 11-3.
Sunscreen.
Hydrate lots
If you have a car/renting one, buy a sunshade for the windshield and use it!! That will cut the heat in 1/2 for your car. Also, bring a blanket or something to put over your baby's carseat because the metal on the buckles can heat up and hurt your baby. One other car tip, misting some water on your a/c vents helps the car cool quicker. Or something like that. It works - try it.
Check out best bottoms diapers for kids if you are going to cloth diaper. They are great for traveling. Don't forget to buy a wet bag from them either. Wet bags are very useful no matter what.
Think about a Character meal instead of M&G. Paying to meet characters and eating seems to be a hot topic, but I find it useful with a toddler because the characters come to you, they are very patient with kids, you can control the situation, you are in a cool place and you get to EAT! And if your toddler is picky, there are lots of choices. And he's free!! Win/win.
Hope that helps. Staying out of the sun is the most important thing though. And using plenty of sunscreen is important.
And did I mention sunscreen?
