That outdoor dining space of Alien Pizza Planet in the Space Mountain entrance corridor is actually AIR CONDITIONED. This is not super well known, but we always eat out there. It's kind of like a very cool cave.
Red Rose Tavern has a large dining area indoors. There is a little room off the side closest to the Pinocchio ride that is an additional seating area.
Docking Bay 7 is actually a pretty small dining room and gets crowded in the summer. It's not a reliable place to find indoor seating.
Plaza Inn has 2 sides and plenty of seating indoors. Most people who eat there go outside to eat.
Flos is the largest one at DCA. Smokejumpers is second largest. You can usually find a seat inside if you walk around a couple times. All other dining locations at DCA are outdoors. Boardwalk Pizza area has covered seating with fans, which usually feels really nice on a hot day.
Craftsman Grill at GCH has a FREEZING indoor dining area, but it's small. Midday, though, there is usually no one there so it's a good option for lunch when you are at DCA.