There's loads to do...
I lived in DC and around DC more or less from early 1979 until the end of last year (in the city, in VA, and in MD).
If you are looking for a market and you go to the Hill, you might check out Eastern Market. It's the last operating market in the city - a nice market with crafts on Saturdays and a big flea market on Sundays. (Easternmarket.net) There are a number of nice wineries in Virginia within a couple of hours. Lots of great restaurants and nice ethnic areas in the city. I know more about the moderately priced restaurants than the super-expensive.
If you are looking for drinks with a bit of a view, go to the upper terrace of the Hotel Washington near the treasury building at the end of the day - very nice!
There are loads of great museums, many of which are free. In addition to the usual fare, the Spy museum is fairly popular and pretty interesting too. The FBI used to run tours, although I don't know if they resumed them completely after 9/11. The Holocaust museum also is something else, although it's a harder visit. In Baltimore, the aquarium is wonderful.
If you like history, the water, and seafood, you might head over to Annapolis. Old Town Alexandria is also a nice nearby town with loads of history (metro-accessible, with a bit of a walk or a quick cab ride down to the river); lots of shops and the Torpedo Factory, where local artists exhibit.
Many beautiful parks to check out. I'm particularly partial to Great Falls on the Virginia side. (But then that's where I'm from!) You can go paddle-boating on the Tidal Basin, but it will probably be way hot to do that, unless you go early in the morning.
The scenery outside the city - especially out toward Loudon County on the Virginia side and out toward Frederick County on the Maryland side is very pretty. There are lots of historic sites out this way (Oatlands in Loudon is a gorgeous house with nice gardens.) Lots of Civil War history, as well, which I must admit isn't my thing, but attracts a lot of attention in the area.
I could go on and on. If you are looking for additional recommendations, let me know. (Can you tell how much I miss it?! I LOVE LOVE LOVE DC!)