The Air and Space Museum has the Udvar-Hazy Annex out near Dulles Airport. It has all the stuff they can't get downtown, and is really great.
I would recommend dinner at the Old Ebbitt Grill. It is on 15th street, one block east of the White House. Old DC charm... Make sure you eat in the old dining room. The *new* dining room is like a Holiday Inn.
You have all the Smithsonian Museums on the Mall. Decide which ones you are interested in, and then go to them, you could get lost in any of them and burn a lot of time... However, the Hope Diamond is AMAZING.
Check out the original documents of America at the National Archives. Actually quite interesting....
The Newseum on Pennsylvania Ave is 4-5 blocks from the White House, and then 10-11 to the Capitol. It has one of the best front porches in Washington.
A walk up Penn Ave from the Capitol to the WH is a study in contrasts.
Where are you going to stay? Traffic can be hideous. And parking expensive, and the Metro is plenty crowded during rush hour. If you have a rental car, then take different routes into and out of town each day. Massachusetts Ave, Connecticut Ave, Georgia Ave, North, East ad South Capitol Streets. You can "see" the different DC's that way.
There in nothing but the Museum Shops and Roach Coachs on the mall to get something to eat. You have to walk two or three blocks to sit down and eat, but if it was my son and me, we would hit the Roach Coaches. There is some real selection there...
Its about a mile and a half to walk from the Capitol to the Lincoln Memorial. Remember that. My wife still calls it the "Death March".....
Really, its about what you are interested in. I like to "imbed" myself in the local culture, eating in local restaurants, walking the streets, seeing the off beat stuff. And playing golf.. There is no finer place to trace the roots of our country, and how it developed.
Rich