Here’s a few more places of interest.

Breakfast:
Krupin’s on Connecticut Ave. DC’s version of a Jewish deli. Best on weekend mornings. All other meals served as well. Occasionally frequented by political talk show types.

Eastern Market. Capitol Hill area, accessible by Metro. Standard city meat/produce market. At least one vendor sells downhome breakfasts and lunch. Gets very crowded.

Lunch:
CF Folks on 19th Street. Shoebox sized joint serving lunch only, M-F. Sit at the counter and order from blackboard, not menu.

Breadline on Pennsylvania Ave, across from World Bank. Lunch only. The only decent sandwiches in town. A little chaotic around the counter but it works.

Maine Avenue fishermen’s wharf. Ramshackle gathering place for purveyors of seafood. Always slated for demolition but somehow it survives. Mostly fish for cooking at home, including bushels of crabs, but also has several vendors selling fried fish sandwiches, boiled shrimp, etc. Fun on the weekends.

Reeve’s on 12th Street. Over 100 years old, though not in this location. Breakfast, lunch, dinner. Comfort foods. Big pies.

Hard Times Cafe. King Street in Old Town Alexandria. Chili joint with good beer list. Cheap. Wander over to Shuman’s Bakery for a jelly cake afterwards.

Five Guys. Fayette Street in Old Town Alexandria, other locations too. Burgers and fries.

Sunday Brunch:
Gabriel on P Street. A bit of a splurge but worth it. Lavish buffet highlighted by a roasted pig on a spit.

Anytime:
Eden Center in Falls Church, VA. All-Vietnamese shopping center. Dozens and dozens of eateries, several specializing in banh mi. Amazing place.

Sightseeing foods:
Nat’l Gallery of Art has a gelatto bar adjacent to its cafeteria. Smithsonian Castle has a weekend brunch by reservation only. Standard brunch fare but the surroundings can’t be beat. Cheap cafeteria atop the Kennedy Center. Stroll the rooftop afterwards for great views. Appropriately, USDA headquarters on the Mall has a farmer’s market on its grounds on Friday mornings.