Well, you've got your inside places and your outside places.
In the Magic Kingdom, there's some seclusion near the rest room in TomorrowLand near the old Skyway. There are some benches on the walkway from Tomorrowland to Mickey's ToonTown, if I remember correctly. There are some good spots in Liberty Square near the Riverboat and also at Sleepy Hollow. There are some benches around Big Thunder Mountain. The exit area at Splash has some spots. Then there is Tom Sawyer's Island. There are a few spots in Adventureland near the Jungle Cruise.
Inside, someone's already mentioned the Exhibition Hall, there are some seats in the Hall of Presidents if you get there right AFTER a show starts. Same for the Country Bear Jamboree.
At Epcot, I'm pretty sure there are some spots inside Innoventions. There are spots inside the Living Seas that come with a view. There are some spots int he post-ride part of Test Track. There are places in the square in Mexico, There's a nice shaded area outside the bakery in Norway. The gardens at China are quiet, but I don't remember if there are any benches. Again, there is seating in the pre-show area. The show area at the American Adventure I think is available if there is not a show coming up. There is seating inside the American Adventure with live enterainment (The Voices of Liberty). This is one of my favorites. There's a wonderful area in France on the canal side of the shops. There's a wonderful area near the back of the British area, again with live entertainment periodically. There are several spots in Canada, near the hotel, near the water and near the gardens.
At Disney's Hollywood Studios, the options are not as plentiful. There are some spots in vairous courtyards, and there is seating near the Big Hat. There are other possibilities near the Sci-Fi Drive-In and near the Muppets.
At the Animal Kingdom, there are a number of choices. Behind Flame Tree Barbecue is an excellent spot. Also around the Tree of Life. There are possibilities near the exits for Dinosaur and Kali River Rapids. There are also air-conditioned spots at Conservation Station.