The Babe Ruth museum is another. And, it is expanding into Camden Station, adjacent to Camden Yards - home of the Orioles. The addition has sports memorabilia from all sports in Maryland - even the NCAA basketball championship trophy that Maryland recently won.
I don't know if the B&O Railroad museum has reopened, but if so, it's also nearby the previous. It was damaged last winter when a large part of the roof collapsed under heavy snow.
Ditto the Drum and Bugle Corps show. The Evening Parade at the Marine Barracks (8th&I Streets S.E., on Capitol Hill) is even better, as it has the Marine Band in addition to the Drum and Bugle Corps and Silent Drill Team. It's every Friday night during the summer months. It is free and no one is turned away at the gate, but unless you call ahead for a reservation you will be shuffled into the end zone seating. Call ahead.
You will want a break from the museums after a couple of days. Rent a car and go out to Antietam, which is the best preserved of the local battlefields. Harper's Ferry is worth seeing too. Back in D.C. we have two showplace churches, the National Cathedral and the Shrine of the Immaculate Conception. The National Cathedral tower has the best panoramic views of the city -- if you're looking for a view, go there rather than the Washington Monument. No line, and a better view.
The Franciscan Monastery is a short jog east of the Shrine -- the Catholic U. neighborhood -- and is also a fascinating place. The guidebooks will talk about the catacombs, but the church itself is also interesting, as it is modeled on a Byzantine basilica destroyed by the Turks. Mount Vernon is a good break from the museums, and you should not miss the Custis-Lee mansion and the Tomb of the Unknowns at Arlington.
The Masonic Temple in Alexandria also has a great view. I neglected this site for years because I had the notion you had to be a Mason to go in, but not so. It's an open building. Incidentally, it sits on the site of the Civil War's Fort Ellsworth, and the traces of the old entrenchments can still be seen from the tower due to the different color of grass in the filled-in sections.
The original Library of Congress building -- the Jefferson building, across First Street from the Capitol -- should not be missed. One of a kind. Last but not least, give your congressperson a call and see if he/she will take you to the top of the Capitol Dome. This may not be available as access was shut down after 9/11, but if it has been reopened, it's a terrific spot. Relatively few people have ever had the opportunity to do this because under the traditional rules you had to be personally escorted by a Member of Congress. I don't know about post 9/11 availability and rules, but it's worth checking on.
They also have a children's museum and Fort McHenry to visit.