Hi, S.. I checked and it looks like your youngest is already out of high school -- so I assume that it's just you and your husband on this trip with no kids along?
Go to the Smithsonian Institution's web site, www.si.edu, to see what's in all the museums that they run (including Natural History, American History, American Indian and Air and Space Museums). Those alone can fill an entire stay. The National Gallery of Art (not part of the Smithsonian) is at www.nga.gov and is wonderful if you love fine arts. The National Portrait Gallery and Museum of American Art (all one building) are great to me, check them out online too. The portrait gallery is at the edge of the very hip "Penn Quarter" area with tons of restaurants and galleries and I would advise walking around there and having a meal; DC's small Chinatown area is right there too.
Spend at least several hours in good weather walking around the memorials -- I'd start at the far end, the Lincoln Memorial, and walk toward the Washington Monument by way of the Korean War , WWIIl, Vietnam and Roosevelt Memorials. The Jefferson Memorial is not on this path but off to one side, still in walking distance. Get a very good map and watch out for cars -- drivers can be awful to pedestrians here!
Now, other cool stuff: International Spy Museum if you dig that. It does cost (about $14 and up) but is well worth it if it's your thing. The Newseum is also good if you are interested in current events and/or history of journalism, costs about $20 a ticket. Not sure about a congressionally sponsored Capitol tour but the Capitol Visitor's Center and the tours that go from there are good.
Ford's Theatre has an excellent and very "doable" small museum about Lincoln and the Civil War if you enjoy that, but be sure to check; the museum is not open if there is a show going on at the theatre at that time. But it's well worth a trip if you have any interest in that period. There's a fee but again-- worth it.
I could go on and on and on. Be sure to eat some great meals here. You will find almost any nationality of food. Penn Quarter is good for many kinds of ethnic food and there is stuff all over the city. Do be sure to ask at your hotel for ideas, too, and ask you hotel if an area you want to visit is OK at night; I have never once had a single problem in DC but it's prudent as a visitor in any big city to ensure you're not going off to a restaurant in an area that turns out not to be a place you want to walk around after dinner.
Check the Washington Post's "Entertainment" button on their web site to see listings for plays and concerts. DC has terrific theatre so consider catching a play, especially at the Shakespeare Theatre (has three locations so be sure you get the right one you want!), Arena Stage, Source Theatre....