Good call on visiting DC, it's a great city. I've lived here for four years now and I'm still not tired of it. It looks like you guys have already been to more tourist-y places so here are some local places to check out.
Adams Morgan - Lots of great bars here, I particularly like Roofer's Union, Tryst (a coffee shop that turns into a bar at night), and Brass Monkey. If you're into records, you should also check out Red Onion Records, they have a great selection.
U St - If you're in DC, you have to go to U St and check out Ben's Chili Bowl. Inexpensive eats and it's a DC landmark. You can also go to Busboys and Poets, a bookstore and restaurant that also happens to be another DC staple. 9:30 Club, probably the best music venue in the District, is close by. Federal, an awesome menswear store that stocks Filson, Red Wing, and a bunch of other American heritage brands is also found here.
Logan Circle - Lots of great new shops and restaurants to check out. Filson opened a new store here a few weeks ago, and you can find the Shinola store close by. I would eat at Churchkey, the Pig, and the Maki shop. The Thai restaurants across the street from Slipstream ( a great coffee shop by the way) are also pretty good from what I've heard.
Dupont Circle - Go to Kramer's, easily one of the best independent bookstores in the area. They have a great selection of books, a fantastic restaurant, a cool bar, live music on Fridays, and they're open till 2am or so in the weekends. If you're in the mood to go out, go to The Boardroom, a bar where you can play board games while getting hammered or Mad Hatter (beware the frat bros though).
NoMa - There's really not much to do in NoMa proper, but I go to check out Union Market. It's super bougie, but they have awesome independent stores, vendors, and restaurants. Great place to go for a slice of artisanal cheese and bread, or go HAM with fresh shucked oysters and local wine.
Eastern Market - There's an awesome local used book store here called Capitol Hill Books. If you like quaint and cozy libraries, then this is the place for you. Cheap used books can be had, but I go there especially for their stock of first edition books. You can easily get lost in this place for the entire day. But if you don't want to be a nerd, there's also a great coffee shop here called Peregrine to go to. Across from metro station, you can go to Rose's Luxury (where Obama went for his birthday) or Ted's Bulletin (try the brunch there).
If you're in a shopping mood, and want to drop some serious dough, Alden, Allen Edmonds, Loro Piana, and a bunch of other high end stores are found near the Metro Center-Chinatown nexus area.
Best of luck in the DC! I know you'll have a blast.