Snow in D.C.? Not likely.
Stuff you MUST see:
Lincoln Memorial
Vietnam Veterans Memorial
Washington Monument
Smithsonian Air and Space Museum (see the Imax, skip the planetarium)
Smithsonian Museum of American History
National Art Gallery
Union Station
National Cathedral
National Zoo (if you feel the Pandas are obligatory - we also have some new tiger cubs)
Arlington Cemetary
Cool/Fun Places to shop or check out
Old Town Alexandria
Georgetown
Adams-Morgan (shops, clubs, bars)
Pentagon City Mall
Capitol Hill (daylight only, thank you)
Optional/cool things no Tourist ever goes to:
Philips Collection (small art museum with great collection)
National Postal Museum (more fun than it sounds)
National Building Museum
National Portrait Gallery
Navy Memorial
Library of Congress
CNN Newseum
You will be staying right on a Metro (subway) line. In fact, if you are coming into National Airport, you can take the Metro to Pentagon City from there - it is a short trip.
With a few exceptions you can see/go anywhere you want on the Metro. Do not drive if you can avoid it, traffic is horrible and parking is worse.
Web sites you MUST check out:
http://www.washingtonian.com (great restaurant guide - including cheap eats)
http://www.wmata.com (subway web site - surprisingly excellent)
Stuff to Avoid:
Never sit down in a D.C. restaurant without checking the prices first. There are a number of places that cater solely to those who only eat on big dollar expense accounts. Excellent but pricey..
Cab Rides: Virginia cabbies like to make big dollars on trips into the city. D.C. cabs do not have meters and use a confusing zone map system. Even residents are frequently not sure what the fare should be.
I live and work in D.C. so email me with any questions you have or ways I can help...