Georgetown - M Street has all the great designer stores. Wisconsin Avenue has Madewell, Zara, Ralph Lauren, Gap. The further North you go, the more local boutiques there are including lots of upscale consignments shops near P Street. For food, check out Pizzaria Paradiso, Miss Saigon for Vietnamese, Mr. Smiths or J.Pauls for American food.
If in Downtown D.C., then by all means check out Metro Center with all the shops listed above. There's also a high end TJ Maxx although it's a smaller store and Nine West, Banana Republic, Ann Taylor, and a decent Macys with lots of designer stuff. For quick food and or if you have kids with you, stop off at the Food Court at the Shops (although no shops anymore).
In Chinatown, there is Loft, Urban Outfitters but more restaurants. MatchBox is the best place to go for wood fired pizza. Get there early otherwise the dinner/happy hour crowd show up and there will be a long wait. Lots of other restaurants down on 7th Street.
