Fantasyland: Snow White's Grotto - I'm not sure when you are visiting, but lately they have had characters in this area. By "tables behind Matterhorn" do you mean the Fantasyland Dock? If so, that is a good place
Main Street: Seating near Jolly Holiday - I wouldn't consider this area calm or quiet. Maybe the upper deck outside of the train station on Main Street.
Adventureland: Tiki Room waiting area - This area is rarely calm or quiet. Adventureland is very crowded.
New Orleans Square: Harbour Galley - good location on the lower decks. They just closed Port Royal and made it a dining area. Since most people don't know about it (yet), it is very calm.
Frontierland: Tom Sawyer's Island - especially the picnic benches facing the Mark Twain dock
Critter Country: Winnie the Pooh area - This is now called Bayou Country and is very crowded since Tiana's opened. Depending on the time of day, the top story of Hungry Bear in the back can be calmer.
Toontown: At the far end of Toontown they have a grassy area that they refer to as a quiet place.
Take a loop around on the train.
Avengers Campus: Tables between Hyperion and Guardians, where Shwarma cart is located
Carsland: Side seating near Flo's
Pixar Pier: Paradise Garden/Boardwalk Pizza eating area, Lower level of SSS.
Grizzly peak: Benches near GRR - more like the backside of GRR.
Hollywood Land- Near the stage, near Studio Catering, Philharmagic waiting area