There ain't too much happening in METROPOLITAN D.C. However, Corey from D.C. may be able to steer you to a couple local tourneys within the city itself.
There is, in fact, a weekly $20-entry-fee 9-ball tournament almost every day of the week in the suburbs, but there is only one that is accessible by the subway (Metro). On Wednesday nights at Orange Ball in Rockville, MD, there is a tournament which usually attracts quite a few of the local pool folk. It is a non-smoking room with a full bar and restaurant. Orange Ball is located about 3 blocks from the subway station in the heart of downtown Rockville, MD. It begins at 8:00 p.m.
Depending on where you are staying, I would suggest that you do not be out on the streets and walking during the late night in downtown D.C.
Here's a rundown of the D.C. metro area weekly tourneys:
Mondays at 7:30 p.m.
Bill and Billie's Cuesport
1244 Ritchie Highway
Arnold, MD 21012
Phone: (410) 544-8185
$20 entry fee, race to 7, double-elimination format
$100 added for 16-plus players
Tuesdays at 8:00 p.m.
Champion Billiards Cafe
904 Upper Fairlawn Avenue
Laurel, MD 20707
Phone: (301) 604-1300
$25 entry fee, race to 7, single-elimination format
Big Rack
Wednesdays at 8:00 p.m.
Orange Ball Billiards and Cafe
430 Hungerford Drive
Rockville, MD 20850
Phone: (301) 309-6440
$25 entry fee, race to 7, single-elimination format
Thursdays at 8:00 p.m.
First Break Cafe
46970 Community Plaza #2
Sterling, VA 20164
Phone: (703) 444-2551
$25 entry fee, race to 5 and race to 3, double-elimination format
Sundays at 1:00 p.m.
Top Hat Cue Club
8809 Satyr Hill Road
Baltimore, MD
Phone: (410) 665-1906
$25 entry fee, race to 7
$100 added for 16-plus players
JAM