Epcot is easy as there are tons of little nooks and crannies in WS. My favorite is what I call the tile room in the Morocco pavilion. There are the gardens in the China pavilion and the tables for the QS in Japan.
At MK there is the porch with the rocking chairs across from the Christmas Shoppe, usually during the day the area where the dessert parties is open to use, there are also some benches behind the Christmas Shoppe, there are table and chairs inside the GS area in Liberty Square, while it is still there lots of shade on Tom Sawyer's Island.
At AK, lots of little paths with little shaded areas with either benches or big ole rocks you can sit on. The area behind Flame Tree BBQ has loads of tables and shade and I've never seen it full.
At HS, this one is tough as there is not a lot of shade in this park. The benches outside of the Star Wars meet and greet are usually shaded, the area in front of Brown Derby has some benches and certain parts of the day is shaded. The little courtyard that used to be for a QS beside HBD has tables with umbrellas as well as the area across the street from it. There is a bench on the porch of ToT before you turn into the lobby room that is under cover, not sure how you would get to it without going through the line for ToT though. There are also some benches off to the left after you exit the ToT gift shop that are in shade parts of the day. Speaking of ToT you can go through the gift shop and there are benches inside the "lobby" area after you exit the ride.