The National Zoo is pretty similar to the Cleveland Zoo (different animals), though the price is much better, Free.
The Air and Space Museum is the favorite of 10 to teens, especially boys.
Contact you Congressman and Senator to try to get tickets for a White House tour.
National History Museum is great if you are into trains, cars, technology, or just a fan of US history.
Natural History is pretty cool, particularly, the Insect Zoo and the Dinosaur Exhibit, if you are anti-Evolution, you might want to skip this though, they are scientists and present what I feel is a fair and straight forward approach to natural history, but that means evolution (I have personally seen/heard people criticize the exhibits with references to evolution, as being full of lies and anti-bible).
The new Native American Museum, is the newest and either gets raves or rants. Several people I have spoken to really dislike the layout of the exhibits. The contents of the exhibits are nice from all accounts, but the lack of logical layout has rankled the feathers of most.
The East and West Galleries of art are pretty nice though generally not well received by children in their early teens or younger.
The don't miss gallery for woodworkers in the Renwick, 17th and Pennsylvania.
The sleeper galleries are the Freer, the Sackler, and the African Art, all of which are very close to the Castle.
The Vietnam War Memorial is not appropriate for children that can't appreciate what it stands for, there are always people present that are morning an loss and visitors of all ages need to respect that it is a solemn place. I have seen the hurt on the face of people mourning the loss of friends and loved ones when children who are disrespectful are present. I personally think it shows how poor some people are at parenting.
The Korean War Memorial, and the FDR memorial should be seen at night, there is no way to explain the difference if you don't see it for yourself.
Take one of the tour mobiles or DC Duck or something, you will get a lot more out of the trip that way.
Union Station or the Old Post Office Pavilion are good places to grab a bite to eat.
The Holocaust Museum is not appropriate for small children, I doubt my kids will be allowed to go until they are at least 12 to 14. The contents of the museum do not always contain all of the information and at times the omissions are significant. The information presented is accurate, with the exception of what is omitted. I greatly recommend that anyone that visits the museum do some independent reading and research afterwards to get a more rounded understanding of the full tragedy of the Holocaust.
Drew