Sure, there are lots of shaded areas around the parks where you can take a break from the heat. For years, our family went to WDW every August and I think the best advice is to get up early, be at the parks for opening, leave around lunch time to take a break (either nap, go swimming at the hotel, etc.) and then go back out for dinner and an evening in the park. If you're not staying on property and think it's too much of hassle to travel back & forth, definitely plan on an afternoon break. Some cool, indoor & quick loading attractions include:
MK - Carousel of Progress, Tomorrowland Transit Authority, PhilharMagic, Hall of Presidents, Country Bear Jamboree, Enchanted Tiki Room, WDW Railroad
EPCOT - Club Cool in Innoventions, Living Seas, Universe of Energy, the American Adventure, O Canada (indoor movie, but you have to stand). You can be inside a lot at EPCOT even if you are standing online (like Soaring, Living with the Land, Maelstrom in Norway, etc.)
AK - It's Tough to be a Bug, Finding Nemo, Dinosaur - the queue is mostly inside, Festival of the Lion King (outdoor but shady; although waiting in line can be a killer), Kali River Rapids to cool off
DHS - The Great Movie Ride, MuppetVision 3D, the line for Star Tours is usually indoors as well
For SeaWorld, we usually go first thing in the morning and do Atlantis and the major shows right away (Dolphins, Clyde & Seamore, Shamu). If we plan it right, we usually beat the crowds and the heat and do most of the major stuff by 1PM. Then we have lunch, relax, and do the indoor pavillions (Sharks, Manatees, Penguins, etc.) when it's really hot. Usually, we're about ready to leave by 4PM. I've stayed for the night time show before and it is good, but for us, it's not worth being so exhausted.