Restaurant Kolumbia is right at the NW corner of 18th and K. While a full meal there for lunch or dinner can be pricy (though worth it), the $9 lunch at the bar is an incredible bargain, and the happy hour menu and drinks at the bar are equally great.
At 18th just below I Street is the terrific happy hour, 3pm to 7pm, at Taberna del Alabardero. Half price tapas and sangria. Well, the tapas start out VERY expensive, but go for the patatas bravas and the pate de conejo. A tremendous one-two punch.
for lunch, try CF Folks or the Well Dressed Burrito. Both are great. For drinks, try out Vidalia's happy hour, they have a fair amount of wines by the glass and free "tastings" every night from like 5 to 7 or so.