U.S. Capitol: I very highly recommend touring the Capitol via the new nearly billion-dollar Visitor Center. Google and reserve tickets for that. The Visitor Center is a museum in itself and has some very interesting artifacts, including a draft of the Constitution. Exhibits rotate.

http://www.visitthecapitol.gov/

The Capitol is quite restrictive on what you can bring in, but not as severe as The White House:

http://www.visitthecapitol.gov/Visit/Visitor%20Safety%20and%20Policies/

Backpack Warning: Do not bring large daypacks, backpacks or luggage into the Capitol. Any bag larger than 14" wide x 13" high x 4" deep is prohibited. Other restrictions include cans and bottles, full or empty.

Newseum: Everyone raves about this recent addition to Washington. It's one block off the Mall. A ticket is required (the Newseum is not government or Smithsonian). This was my nieces' favorite place. Worth Googling. A neighbor of mine was down there last week and said there is a Kleenex dispenser at the 9/11 exhibit.

Spy Museum: Also requires a ticket.

Memorials: I think that especially in summer, the Lincoln, Jefferson, Washington, Vietnam, Korean and World War II memorials are best seen at sunset and after dark. Or very early in the morning -- sunrise. The Mall cools off appreciably because the grass does not retain the daytime heat like the asphalt and brick neighborhoods do. There is usually a breeze at night on the Mall.

Arlington National Cemetery: If you've not been, I very highly recommend. The cemetery is very welcoming of visitors (it's even dog-friendly), fascinating and sobering. There is a parking garage. It's about a mile walk from the Lincoln over Memorial Bridge. There's a Metro stop or you can Tourmobile.

That's about it off the top of my head. Please feel free to PM me about your trip.