One place I wish I had taken more advantage of during my years in DC is Eastern Market. Situated in the historic Capitol Hill neighborhood (which has come along way since my tenure in the city), Eastern Market is a public market that offers a wide range of fresh food including meat, seafood, cheeses, produce, bakery items and fresh flowers. On the weekends you can find farmer's markets, arts & crafts vendors and live music. They even have a flea market on Sundays.
There wasn't too much going on when I visited as it was a Thursday afternoon. It was still quite enjoyable to peruse the cases brimming with gorgeous foods. The bakery items alone made you stop in your tracks and I won't even mention the beautiful flower stand;)