Lebanese Taverna is probably my favorite restaurant in Washington, D.C. (There is another location in Arlington, VA, if you're out that way.)
Located in Woodley Park, near the National Zoo, Lebanese Taverna is literally across the street (Connecticut Avenue) from the Metro (Red Line, Woodly Park-Zoo/Adams Morgan station).
The restaurant has a nice atmosphere, wonderful service, and a great and extensive wine list – there are many options for by-the-glass and the vintages are from all over the world, including Chile, Argentina, France, Italy, Lebanon (natch), California, and Oregon. While I go for the carnivorous options, this restaurant was recently rated as D.C.'s best vegetarian restaurant. I'd highly recommend the lamb kebob for those of you who are meat-eaters. It is succulent and delicious; has mushrooms, tomatoes, onions, and green peppers; and is served with rice, tomato sauce, and sumac-spiced onions. Or, instead of entrées, you can order a number of different mezza plates and have a tapas-like dining experience.
LT is great for a group of friends, business associates, dates, or even a group of travelers. It's listed as "casual," but most dinner patrons are dressed smartly. (Don't wear a tie or dress up unless you want to; you can get by with nice casual clothes.) This location has a nice outdoor patio with large umbrellas that is open when the weather is nice.
Metro: Woodley Park/Zoo-Adams Morgan (Red line)
Web site: http://www.lebanesetaverna.com/