Another vote for Les Halles for breakfast or a snack at off hours. We love love love the pastries, cheese tray, salads, sandwiches, lobster bisque...basically everything there! Even during busy times (dinner and lunch) we've been able to find seating.
This isn't a food tip...but if you enjoy a beer, a stop at Rose and Crown Pub in Epcot is a must! My siblings and I do a group trip every year, and on evenings when we're wandering around World Showcase at Epcot, we start at R&C for a great go-cup of one of their pub blends (half and half, aka half Harp and half Guiness; Golden Fox, aka half Boddington and half Bass Ale; etc.). We find the prices there as good as at beer booths with servings a bit larger for the money spent.
When visiting AK, our first stop is at Kusifari for a pastry or breakfast sandwich and coffee. It's a little tucked out of the way, and it's tables (tall ones that you stand around) are tucked even further to the side...so we can usually find an empty spot to gather (in the shade no less) even when the park is crowded.
Columbia Harbor House in MK isn't a hidden gem...but you'll often find lots of available seating upstairs when downstairs is packed.
And just about any deluxe bar/lounge attached to a signature restaurant will offer much of the same food in a relaxed setting...without an ADR should you decide last minute to try something different than QS and the standard TS places.