I'm planning afternoon breaks at the HOJO and plenty of good food, but I welcome any other ideas, including how to cope with the heat.
You guys are going to have a great time! I'll address the heat issues, as I'm a 30-something with a heart condition which has caused heart failure & find heat to be one of my biggest triggers to feeling bad, so I've gotten pretty good at navigating my way through my visits to stay cool despite the heat.
My first suggestion is always to have an option for a quick retreat to a nearby hotel to cool off in the hotter parts of the day & you already have that covered!
My second suggestion is to familiarize yourself with the areas with air conditioning throughout the park which might cool you off when needed. For example, I usually retreat to visit Mr Lincoln at least once during the warmer part of the hours I'm in the park (and I also appreciate the air conditioned lobby area before the show with all of the displays with historical memorabilia, etc., and have been known to duck in there and cool off before heading back out), I also have been known to hit up the Main Street Cinema for the same reason. Depending on how many groups/families decide to do this same cool off method, it might get a little overwhelming, but usually I've found them to be pretty quiet and a good place to step away from the overwhelming crowds. I also like to hit the air conditioned shops at non-peak shopping hours (i.e., just before or after lunchtime, up to about an hour before any afternoon parade tends to be somewhat uncrowded) for the same reason.
I also sometimes schedule air conditioned snack or beverage breaks at non peak times for the same purpose...for example, The (air conditioned) Stage Door Cafe tends to be pretty deserted when it first opens, and it is a great place to cool off & get away from the crowds while drinking a quick soda. I think it opens around 10? I've been in there several times in the first hour to hour and fifteen minutes it is open & found that a great spot. As I am vegan & on a low sodium diet, dining options in the park are limited for me & I don't have a ton of feedback there, but I bet others can give good feedback on which spots have a/c and when they are less busy.
I also make sure I hit rides with any lines/waiting areas in direct sun long before the hottest time of the day. If heat sensitivity for any member of your party is an issue, i'd advise this, too. For example, I know the wait with DAS at Small World isn't long, but even a few minutes in the direct sun there can make me miserable, so I tend to hit it early or late & avoid it during the 10:30 AM to 5:30 PM timeframe. I know, as a teen, he probably thinks he's too cool for rides like that, but that gives you an example. My suggestion would be to really look at the ride loading areas for DAS for the rides you want to hit on a map to familiarize yourself before the trip, then when you are there, scout out those locations when touring around the park and get a feel for how hot they might be & hit up the hotter ones early or late.
Usually I suggest the train as a good sit down & rest opportunity during the hotter hours you are in the park, but sometimes I can get a bit hot on that during the warmer hours, so I tend to use that to help me get around when it is getting a little warmer, but not too hot. If stairs down from the train station are an issue, the Main Street station isn't a good option, but if all members of your party can navigate going downstairs easily, I'd suggest using the train to help you get out of the park for your afternoon hotel rest on at least some of the days...I cannot tell you how much I have missed having the option to take the train up to the main gate to help me get out of the park while the railroad is down! I often take the train from the Toontown or Tomorrowland stop up to Main Street & exit the park for my afternoon rest that way. It allows me to rest a bit before the walk back to the hotel, which is greatly appreciated. And, FYI, I think I heard that at WDW the monorail trains are air conditioned? Don't expect that at Disneyland. I've heard the front and rear cars with the drivers have a/c, but have never experienced that myself. If, at any point, your plans were to take a nice, air conditioned ride on the monorail to Downtown Disney for lunch, be advised it will be hot (and, depending on how hot it is, the monorail might actually be closed due to the heat!).
Other suggestions to beat the heat...the obvious ones apply, like bring water bottles, wear hats and/or have cooling towels, plan to buy cool treats like Dole Whip or frozen lemonade or ice creams (in DCA I've found that Schmoozies in Hollywood Land makes really refreshing cold smoothies--I get them without the dairy and I swear they are even more refreshing with just the blended frozen fruit!). As you tour the parks, make mental notes about where there is good shade and seating.
As I am flying in from the Eastern time zone, my body wants meals at non traditional times, so I tend to eat lunch and dinner closer to when I would eat at home and I've found that it is surprising just how much cooler most of the restaurants and quick service eating areas are when there are very few people eating there. That said, as you tour the parks, keep an eye on the seating arrangements for any places you want to eat (especially the quick service spots!) and see how hot the eating areas might be. For example, the Cozy Cones in Carsland are awesome, but the seating there is so hot, even the seats under umbrellas. I tend to either stop there to grab a quick pomegranate limeade and walk to shady areas elsewhere, or I hit them later in the evening, when it is much cooler.
I hope the Fantasmic dining package is up and running by the time of your trip! And I hope you guys enjoy your trip. The people here on the DL boards are great at answering questions or giving suggestions, please feel free to hit us up if you want/need any help or feedback!
~Meg