Depends on the park. For instance, at MK there are tons of tables at the usually-closed Tomorrowland Terrace, outside the Lunching Pad in Tomorrowland, Cosmic Ray's, Hall of Presidents, or along the fence at the Plaza Ice Cream Parlor. In Epcot, there are lots of benches in Future World and tons of tables inside the Land. In World SHowcase, there's tables next to Rose & Crown, in Morocco, outside America, Germany, China. Upstairs on the pyramid in Mexico is a great place to eat - there are a few tables and you're up high which is great for people-watching. DHS has tables on Sunset Boulevard, around the water near Dinosaur Gerties area, outside Backlot Express, plenty outside of Pizza Planet and Studio Catering.
I believe anything outside is fair game. Going inside is okay if it's not busy, I do believe paying people have rights to the tables when you buy food there.