Marine Barraks has a formal retreat parade on Fridays. There is a Regiment assigned to the Barraks which handles the Marine Corps' ceremonial duties in D.C. Each Friday one batallion from the regiment conducts the parade. It was the most impressive thing I've seen in three trips there. The Marine Band, The Marine Drum & Bugle Corps and Silent Drill Team all participate. At one point they have herald trumpets in the parapets playing taps with spot lights illuminating them. The whole parade is a fantastic and impressive event. When you arrive, each car is greeted by a Marine in full dress uniform who helps all women out of your car and personally escorts them to their seat while you go park. You have to have tickets because they only allow as many spectators as they have seats in the bleachers. We got tickets through a convention I was attending, but I'll bet if you called the Marine Barraks when you got there, they could arrange tickets for you.
Another "don't miss" is the Tomb Of The Unknowns at Arlington National Cemetery. Be sure to stay around until they have a changing of the guard. As best I remember, they change the guard every thirty minutes, so it's not a long wait even if you just missed one when you arrive. The Tomb guards are from the Army's "Old Guard" Division. Most of the military ceremonial duties in Washington are handled by the Army and these are the guys who do it. They are unbelievably good at it. Except that some are white and some are black, they all look and move the same. It's almost like they came out of molds. While we were there someone told us that everyone in the Division is between 5' 11" and 6' 2".
Like others said, the Smithsonian is definitely worth a visit. They have THE Star Spangled Banner on display there, the actual flag that was flying over Fort McHenry and that inspired Francis Scott Key to write the words. The flag is enormous and every thirty minutes, they dim the lights, illuminate the flag with a spot light and play the National Anthem. It's about as close to absolute silence by a crowd as you will ever hear.
I would consider The National Archives another must. They have the actual Declaration Of Independence, the original Constitution and many other historical documents on display. The line is usually pretty long and the wait can be annoying, but it is worth it when you finally get in and realize you are looking at the real thing and not reproductions.
When you go to the Vietnam Memorial, I suggest you go right at dawn. There are few people around then and seeing the monument at that time and without the crowds creates a really special atmosphere. Viewing it at dawn will make the visit to the wall one of the most moving experiences you will have in your life.
I also enjoyed the tour of the Capital. If Congress is in session, you can go to your Representative's or Senator's office and get tickets to sit in the gallery. We did this and it was pretty interesting, though somewhat boring as all they were doing was reading stuff into the Congressional Record. I think it would be pretty neat though if you happened to luck out and were there when there was debate going on over an interesting bill.
There is so much to see there that you'll have to bust your rear to get even part of it covered in a few days. Like ComeFrom? said, wear comfortable shoes
