If you drive you can get reasonably good parking spaces near the mall before 10 am, but beware that parking tickets are the only things DC government does well. The cafeteria at the American Indian museum is the best of all the museums. The only things worth seeing in the Hirshorn are in the basement. There is a room to the left when you enter the new building of the National Gallery of Art that alot of people miss but contains wonderful small works of art. The view from the tower of the Old Post Office is almost as good as the Washington Monument and there is almost no wait. There is a Ben and Jerry’s there. There is a place to buy half price tickets for local shows there as well. The museum of american art/national portrait gallery is very good and is across the street from a metro stop. Both the Supreme Court and the next door Library of Congress are beautiful inside. The peacock room in the Sackler Gallery is very pretty. DC has the best Ethiopian food in the country and Seven Corners in Fairfax county has the best Vietnamese on the east coast. If you are serious about finding the best to eat check out Don Rockwell’s message board. The Albert Einstein memorial which is across the street near the Lincoln Memorial has a really cool statue of him that you can take pictures of your kids climbing on. The national cathedral is beautiful and has nice gardens where you can eat a picnic lunch. The Zoo is nice but very hilly and is harder to get around in because of construction. If you are into politics, many of the think tanks have lectures about public policy which are free.
The weather in October is generally very pleasant, and early winter is easier weather than Jan.-Feb., which can get icky. Note that much of the city shuts down even for the lesser-known federal holidays, like Columbus Day and Veterans Day.
Skip the White House tour. Long wait for only a few boring rooms. You don’t get to see the Situation Room, for instance. The Capitol is a much more interesting tour. If you visit at Christmastime, the Capitol tree is also more interesting than the White House tree.
The Monument is a nice view, but there’s nothing else in it to recommend going up.
You can’t go wrong with any of the Smithsonians, but be warned that food/snack/drink options around there are quite limited. Bring some powerbars and bottled water.
The Holocaust Museum is an unforgettable experience, but it demands a certain internal resolve that may be at odds with a light-hearted vacation experience.
The National Geographic museum might be fun. There are also some newer, themed museums, like the Spy Museum, and the Crime Museum.
The area around the Verizon Center (the downtown sports/concert arena) is called Gallery Place, and it’s very lively with restaurants and bars, and it abuts our little Chinatown.
If you do have a car, it can be diverting to drive along Embassy Row, to see the old, stately buildings, with a profusion of foreign flags. There is also a newer embassy enclave in an neighborhood called Van Ness, and those buildings are generally futuristic, lavish, and visually stunning. You generally can’t go inside, but they’re striking just to see.
And if you ignore or forget everything else I say, remember this one thing: If you ride the Metro, stand to the right on the escalator. We locals will sigh, snarl, nudge, and generally hate on anyone who blocks people from walking down the left side. It’s the one thing we all agree on here. 
As other people have mentioned, most of the Smithsonian dining options are ehh at best, but there’s an exception to that, and that’s the cafeteria in the National Museum of the American Indian. It’s got the best food of all the Smithsonians, and if you’re down on the mall doing the museum thing, stop in for lunch.
That’s true. There was a Subway, Starbucks, and even Fuddrucker’s when I was there last (pic). At least the synagogue around the corner’s still there.
Nows I learns. :smack:
Don’t forget the Ruby Tuesday and Legal Seafood. I joke with a friend of mine that my goal is to try every Chinese restaurant in DC’s chinatown. I figure if I give a few years, I can accomplish it in two weekends.
And there’s a 24/7 IHOP right outside the Ballston Metro Stop. (I know, VA not DC, but it is 24/7.)
And a hell of a hike. A lot closer is the original Bob & Edith’s Diner, a short bus ride down Columbia Pike on the 16 routes from the Pentagon Metro station. (God, how could I have forgotten this?) They’re 24/7 and breakfast anytime - and probably the best greasy spoon in the Mid-Atlantic.
That would be the tastee diner in Silver Spring 
(walking distance from the metrostation)
If you fly into BWI, there is a Metrobus (B30) that will take you from the terminal to the Greenbelt Metro station for $3.10.
As far as museums are concerned, I’m fond of the National Gallery of Art, the Sackler gallery (Asian art), and the US Botanical Gardens, all on the Mall. If you do get a car–or are an intrepid bus rider-- the National Arboretum has a nice bonsai collection, but it’s kind of in the middle of nowhere. The Spy museum is fun, but bit pricey. I was very disappointed with the new American Indian museum, but hey it’s free.
Dining near the Mall, we always used to go to the Old Post Office Pavillion food court for cheap tasty Indian, not sure if it’s still good (or there) though. Union Station is another option near the east end of the Mall.
I ate there in June and it was still there and still very good!
Good to hear! Mmmm, masala dosa…
The National Air and Space Museum - nirvana for an airplane and spacecraft buff.
The National Portrait Gallery and the National Gallery of Art - to see, up close, amazing paintings and sculpture you’ve seen many times in books.
The U.S. Capitol and Supreme Court - just across the street from each other; both steeped in history. You may be able to hear actual oral arguments in the Supreme Court if you get there early enough. There’s going to be a new, big visitors’ center at the Capitol, but I don’t think it’s opened yet.
Walk along the Mall. Nothing like it.
Visit Lincoln’s summer house at the Anderson Cottage, just a few miles north of the White House.
Arlington National Cemetery. If you approach it in the right frame of mind, it will blow you away.
If you go to Mount Vernon, George Washington’s stately, well-kept home just down the river (well worth a visit), PM me. I have a modest request to make.
The Ihop is a few blocks from either Ballston or Virginia Square on the Orange Line. Bob and Edith’s requires either a car or a bus. Something else to think about is that the Metro won’t closes at 3:00 on the weekends and won’t reopen until 7:00am.
Don’t get me wrong, I would much rather eat at Bob and Edith’s because the food is much better and it is one of the last remnants of the old Arlington but it will be harder to get to from the City. Thanks Olentzero, now you have given me a craving for Bob and Ediths. I may have to go there tonight. 
BTW, Olentzero, I agree with you on the Saloon. It is a great place to go with friends and just get a beer in a relatively unpretentious place that won’t be overrun with people making asses of themselves.
Realistically, if you are staying in DC and have been drinking in DC, the Diner in Adams Morgan is the best option.
You can also get pizza at a place in Adams Morgan that is open after last call. I think Julia’s Empanadas in Adams Morgan will be open although it has been a while for me.
There’s Osman & Joe’s Steak & Egg Kitchen a short walk from the Tenleytown metro. It’s a tiny (counter only) diner popular with students and other night owls.
I may be remembering the distance to the IHOP incorrectly. Last time I went there was 10+ years ago, I was drunk, and being driven.
Is Caleb still running the bar? Tell him JS said hello (cf name on the wall reference from earlier post).