Make sure that you visit the Air and Space Museum in town by the Smithsonian AND the new one out at Dulles where they have the Enola Gay, an SR71, space shuttle Enterprise and many other HUGE aircraft in a monster hangar complex...
One suggestion I'd make - leave the car at the hotel. Traffic is hoorible & it's not needed.
The city runs a trolley service with stops at all of the major (and not so major) locations.
Costs about $25 for the day as I recall.
You can get off at any point & then get back on whenever the next trolly comes around which is about every 20 minutes.
They stop going around at about 4 or 5 so you need to be back at your start point by then.
Save the car for night time.
Because you NEED to go back & see the monuments at night - absolutely outstanding.
WW2 Monument is breathtaking & somber at the same time.
Korean War Monument at night is nothing short of errie. It features a typical Korean field with a life size squadron of soliders moving through it. At night it's softly light & you'd swear they were reall living breathing troops. You keep waiting for them to move.
We were there for 5 days over Easter a couple of years back.
Left the hotel by 8 & never made it back each day until well after dark.
Very impressive town.
Instills a real sense of patriotism & gives you a real sense of what this nation is about. Makes you proud to be an American all over again.
Every American needs to go to Washington at least once.
I agree with Wader. The Hop on/Hop buses are the way to go. You can stay on and get a guided tour but you miss going in some places like the Smithsonian. I travel to DC about 10 times a year for Business. Parking and cabs can be expensive and the city is spread out enough that you really need a mode of transportation other than your feet.
Vietnam memorial, seeing all those names and the people scratch tracing their loved ones names onto paper is very powerful indeed.
I'm not sure if they still do it but a tour of the treasury would be cool too.