Well, get yourself downtown to DC--by cab or by the Washington Flyer bus to the Falls Church West metro and get yourself down to the Smithsonian Metro stop. At the Washington Metro website
you will find the itinerary as follows (they gave me today's local times):

Rail : Departs from Board Arrive

WEST FALLS CHURCH METRO
at 5:43pm ORANGE LINE towards NEW CARROLLTON SMITHSONIAN METRO STATION
at 6:08pm


Fare Total (Senior/Disabled) : $2.70 ($1.35)



Itinerary #2

Rail : Departs from Board Arrive

WEST FALLS CHURCH METRO
at 5:43pm ORANGE LINE towards NEW CARROLLTON ROSSLYN METRO STATION
at 5:58pm


Rail : Departs from Board Arrive

ROSSLYN METRO STATION
at 6:07pm BLUE LINE towards LARGO TOWN CENTER METRO STATION SMITHSONIAN METRO STATION
at 6:17pm


Fare Total (Senior/Disabled) : $2.70 ($1.35)



Itinerary #3

Rail : Departs from Board Arrive

WEST FALLS CHURCH METRO
at 5:43pm ORANGE LINE towards NEW CARROLLTON FEDERAL TRIANGLE METRO STATION
at 6:06pm


Walk 0.42 mile S to SMITHSONIAN METRO STATION directions
Fare Total (Senior/Disabled) : $2.70 ($1.35)


At the Smithsonian site, you have access to all of the museums and there is no admission charge. They do not open until 10 a.m. A favorite of tourists is the Air and Space Museum. But all of the museums are great. The Smithsonian stop puts you at the mall between the Lincoln Memorial (absolutely worth seeing--if it is too far to walk, take a cab) and the Capitol--which you can see from the mall and it is an easy walk (you can't get into the Capitol any more except on a tour and you will need a prearranged tour if you want to go on that day). The Vietnam Memorial is right next to the Lincoln Memorial. The White House is a bit of a trek, but it is a short cab ride as well. If you still have time, one of my personal favorites (a short cab ride away from the Lincoln Memorial or a long walk) is the Franklin D. Roosevelt Memorial, which is absolutely awesome and a wonderful testimonial to one of our greatest Presidents.

Personal recommendations: Absolutely don't miss the Lincoln Memorial. Most locals consider it the most awesome sight in the capital. The museums (which ones depend upon your taste) are great. And at least get a glimpse of the Capitol (my favorite, but then I write about the Congress).

Saturday should be brisk, ranging from about 7-10C (or 44-48F). And to act like a local for a day, don't call the capital "Washington" or "DC." Locals call it "the District." Enjoy yourself--and good for you that you are not wasting all of that time at IAD. It is one of the least appealing parts of our nation's capital.