I'd go to the Dulles annex of the Air and Space Museum. They have a Space Shuttle and an SR-71 Blackbird, etc.
I'd visit the National Zoo to see the Pandas.
Both of the above are cool for kids.
Mount Vernon has very new interactive exhibits on George Washington.
If you like US history, the Antietam and Gettysburg Civil War battlefields are interesting and Harper's Ferry is very near Leesburg. Baltimore has Fort McHenry, an aquarium, and a kid's discovery Science Center that is fun.
Of course there are the museums at the Mall in DC. The National Gallery of Art and the Freer are great. The gemstone exhibit at the Natural History Museum is spectacular.
DC in July is hot and humid, get ready. Have fun
