The tostada at the Mexican place by Thunder Mountain is very good and huge and a great deal at $10. It feeds two easy.
The soup in the bread bowl is also a good deal.
Skip the italian place at DTD (don't know about the TS but we did the CS) next to the pretzel place I believe. It was AWFUL!!!!!!!!
I also have one strange favorite I love the "bar/restaurant" that is in the center of the DTD walkway. They have a mostly appetizer style menu but we found it excellent and good for the price. Typically a group of 3-4 of us gets their yummy Fries, Pizza and hummus platter and have plenty of food for all. It is also a great place to people watch and great to get a late bite to eat too.
Wine Country Tratoria in DCA is a sitdown but has great prices and really good food. Usually taking walk ups too. My sandwhich is about $9 with bread and small side.