You could probably spend a week walking around the National Mall and not see everything there is to see. It's a target rich environment and it's made for a tourist on foot. Start with the museums maybe. No matter what your plan is there's a good chance you're going to get sidetracked, which is good. One thing that surprised me was that the White House is just a few steps away from downtown Washington! I guess I always thought the government of the United States existed in some kind of land bubble.
Arlington, Smithsonian, Holocaust, Watergate 
The mall is best at sunset , especially the Korean Memorial..... YES to Arlington and changing of the Guard, The Air and Space Annex out by Dulles Airport is the better one to see if you like looking at Planes, Willey Post's plane and first pressure suit is on display.... The NRA museum has a really good gun collection so does the FBI museum...
I posted the same question here back in March. Got some good suggestions but not enough time to see everything I wanted to see.
Arlington Cemetery is a must. National Mall after museums close. Air & Space is good but American History was my favorite. They are so large and don't open until 10 a.m. it's pretty tough to take in more than one museum a day. National Archives is good but security is like the airport and takes a while to get in. Metro Rail is easy to use but wear comfortable shoes. Contact Senator Inhofe's office to set up a VIP tour of the capital and if you give them enough heads up you could get a White House tour as well.
Eat at Ben's Chili Bowl and really I enjoyed Old Ebbitt Grill near the White House.
Arlington Cemetery is a must. National Mall after museums close. Air & Space is good but American History was my favorite. They are so large and don't open until 10 a.m. it's pretty tough to take in more than one museum a day. National Archives is good but security is like the airport and takes a while to get in. Metro Rail is easy to use but wear comfortable shoes. Contact Senator Inhofe's office to set up a VIP tour of the capital and if you give them enough heads up you could get a White House tour as well.
Eat at Ben's Chili Bowl and really I enjoyed Old Ebbitt Grill near the White House.
