As others have mentioned, the area from Rosslyn down to Ballston sounds like it's pretty much exactly what you're looking for. It's fairly dense with a lot of restaurants, stores etc., a lot of high-rise apartment buildings, and it's a pretty quick Metro ride into D.C. and thanks to the Silver line it's now a fairly easy trip to the Tyson's mall complex as well. The area has a big concentration of young professionals working in NoVa and D.C. and that is reflected in the vibe of the place. It's sorta 'urban' but without the urban edge or grittiness.