Whoo boy. This post is out of control and I apologize. I've probably lived in this area for WAY too long. I'm including the inner suburbs on the list because sometimes, given the rents in DC, you can find equally interesting things there.
OK Go is playing at the Black Cat on Saturday. The show starts at 9:30.
Meeps also features a small but awesome section of reasonably priced local designers: 1520 U Street NW, 12-8 pm on weekends
Georgetown Flea Market: Sundays starting around 7 AM, weather permitting, Wisconsin Ave., just north of S St.
Polly Sue's Vintage: 6915 Laurel Ave., Takoma Park, MD
Takoma Underground: 7000 Carroll Ave., Takoma Park, MD
Rerun: 7001 Carroll Ave., Takoma Park, MD
Knit + Stitch = Bliss: 4706 Bethesda Ave., Bethesda, MD
Suttle Thoughts plays an interesting fusion of jazzy R&B and Go-Go (the DC flavor, not the plastic boots kind), from 9 PM - 3 AM on fridays, they play at 94th Aero Squadron restaurant (5240 Paint Branch Parkway, College Park, MD), barbecue and drink specials --
Bohemian Caverns (2001 Eleventh Street N.W, right off U Street): Shirley Horn, Duke Ellington, Pearl Bailey, Miles Davis, Aretha Franklin, Ramsey Lewis, Less McCann, The Supremes, Roberta Flak, John Coltrane and Eric Dolphy, JFK Quintet, Ella Fitzgerald, Billy Taylor, Dizzy Gillespie and Billie Holiday have all played here since it opened in 1926
Cakelove: 1506 U St. NW - mmm, tasty cupcakes from scratch
Ben's Chili Bowl: 1213 U Street, N.W. - a DC landmark, open since 1958
Ethiopian restaurants on 18th St.: You can't go wrong with one of these. My favorite is Meskerem.
Habana Village: 1834 Columbia Rd. NW, fun little bar/salsa club
Wheaton Library Book Sale, Wheaton Plaza edition, has an entire mid-sized room jammed full of records for $1 each (11160 Veirs Mill Rd, Silver Spring, MD). The store is a little hard to find, it's tucked away right outside the enterance to the food court on the lower level.
Visions Cinema (http://www.visionsdc.com/dyno/index.asp), AFI Silver (http://silver.afi.com/afi_web/), E St. Cinema (http://www.landmarktheatres.com/market/WashingtonDC/EStreetCinema.htm) or The Uptown (http://www.thehoya.com/guide/082401/guide6.cfm) for movies. An added bonus of The Uptown is a visit to the Yenching Palace (http://www.washingtonian.com/dining/Profiles/yenching.html), the oldest Chinese restaurant in the area and the place where they met to end the Cuban Missile Crisis.
In general, in terms of nightlife, it's hard to go wrong in the U St. area. Adams Morgan (slightly northwest) is fun, but way more crowded. Georgetown is overrated and too expensive, in my opinion.