Some of my favorite restaurants:
*Two Amys. Best pizza in town, bar none. The decor is unassuming, the pizza is as authentic as Italian pizza can get. It is a thin pie, lightly coated with olive oil, and it is baked in a brick oven. The ingredients are authentic. It is expensive for pizza ($10-13 for a ~10" pie, I eat a whole one myself), but it is always packed--rightly so. It is located off of Wisconsin Avenue, right past the National Cathedral. It is closed on Monday evenings.
*Moby Dick's House of Kabobs. There are several restaurants of this local Persian chain, but the kabobs are relatively cheap ($6-9), the food is great, and everyone apparently speaks Farsi. My buddy always gets the kubideh, which is a grilled Middle Eastern sausage. I always get the joojeh (grilled chicken breast). I recommend that you get it with the bread cooked in the tandoori oven, not rice.
*Old Ebbitt Grille is a famous tourist attraction, but most don't know it has half-priced oysters after 10 or 11 pm. Great selection of oysters, to boot.
*Sam & Harry's has the best steaks in town, and you'll pay for it. It offers huge sides and is centrally located in downtown. A cheaper steak joint (and a better value, IMO) is Ray's the Steaks, which is in North Arlington.
*Next door to Ray's the Steaks is The Village Bistro. I've always had a great meal there. (www.villagebistro.com
For most sit-down restaurants in DC, you need a reservation on the weekends. I recommend planning in advance. A few good Web sites for restaurants recommendations are washingtonian.com and the food section of washingtonpost.com.
*Get a drink on the roof of the Hotel Washington, 15th Street NW. On a clear night/day, you can see pretty far. They might have a canopy out there with the bad weather.
*Another nice view: on top of the Old Post Office Pavilion on Pennsylvania Avenue, but that would be a daytime activity. I believe it closes at 5 pm.
*Another great (but unheralded) view is from the Iwo Jima Memorial in Arlington. You get a panoramic view of the downtown monuments at night. Dress in layers at this time of the year.
*See a movie at the Uptown, on Connecticut Avenue. It's an expensive endeavor ($8-10), but its a retrofitted theatre with great seats, a great sound system, and a huge screen. You may have to wait in line to get a ticket on the weekends or for popular movies.
*People watching/window shopping on M Street in Georgetown. Priceless.
*The Wizards and Caps might be in town. Neither is a particularly good team, but the Caps have 21-year-old Lemieux in Alex Ovechkin. He's a legit stud.
*If there is any game you want to see in the world (no kidding), Summer's in Arlington has it on TV. They have like 10 satellites on the roof and can get any sporting match from across the world. It is open 24 hours a day.
*Although you didn't want to go the Mall, there were a few exhibits worth mentioning at the National Gallery of Art. There is a still life exhibit of Pieter Claesz, one of the Dutch Masters, and a photography exhibit of the four Brown sisters (see http://www.zabriskiegallery.com/Nixon/TBS/nixon.htm
) taken over 31 years. It is an artistic way to visually see the aging process and how relationships among close relatives can change over time. I enjoyed it a lot but it made my wife depressed, FYI. The best part is that entry at all the Mall venues is FREE.
*When kids are involved, the air and space museum is always a winner.
*If you are into politics, you can ask you congressman/congresswoman for tickets to the Capitol. If you are coming in December, it is unlikely Congress will be in session, so it probably won't be worth your time.
*The White House is always immaculately decorated for Christmas, and is open for the public to see. I actually don't know how to get tickets or what the restrictions are on when the public can see it. . . But it is well worth a 10-minute Internet search. Again, you might want to ask your congressman/congresswoman if they have tickets.
I can provide more specific recommendations, depending on your dietary preferences, budget, where you will be staying, if you have a car, etc.