MK -- benches by the restrooms/City Hall, Tiki Room, sitting area near Splash/under train station, Gaston's fountain isn't very shady but I enjoy the scenery, PeopleMover, Carousel of Progress, Tomorrowland Terrace if not seasonally open, the train or monorail loops.
Epcot -- anywhere inside the Living Seas, jumping fountains outside Imagination, Japanese gardens, the back courtyard in Morocco, gardens in England, down near the movie in Canada.
HS -- My usual quiet rest area is now under construction. Umm... courtyard near the exit of ToT (Tower of Terror), maybe? The park is in such a flux at the moment it's difficult to tell where anything will be good. See if they'll let you sit in the tables outside Studio Catering Co. or the Brown Derby? Might be some benches near TSMM (Toy Story Midway Mania).
Animal Kingdom -- take Rafiki's train to the planet watch, Nemo show, there should be some benches with a fountain near Kali River Rapids.... someone else probably has better insight since I'm usually in and out of the park due to allergies.