Great Question
AK, I like to grab a bite, and relax at the tables outside of Flame Tree.
DHS, the backlot express is a nice place to sit, and have a drink, and watch the people go by . . . then again, with everything past that being torn up, I guess there aren't a lot of people going by there anymore.
MK, Tom Sawyer Island. The checkers table outside the candy shop in Town Square. The little side street to the right on Main Street just after Uptown Jewelers, the little covered walkway in Adventureland between the sunglass shop, and the rest of the gift shops, where the Sorcerers of the Magic Kingdom portal is. Outside the Tangled Restrooms. The porches in Frontierland.
EPCOT, this park is just full of little nooks and crannies. The benches / walls between Spaceship Earth, and the restrooms / locker area. Often there is shade here, and it is a bit cooler. The breezeways on either side of Future World, going through the Innoventions buildings. There are benches, and often a vendor selling drinks. All along the World Showcase there are spots near the edge of the lagoon with benches, and often even shade. Mexico, on the wall of the fountain. I love to sit there while my wife and daughter shop. Norway outside of the bakery. Germany, in the quick service, love to have a frankfurter and a drink. China there are benches around the pond / gardens that are a lovely place to sit for a bit. France, along the fountain, or in the back near the bakery. Japan, behind the koi pond, in the garden area. UK in the gardens.