We just got back from there!!!
My advice: Use the parking garages. We used one and it cost us 18 dollars for the whole day. There is no on street parking until after 6:30 and we couldn't figure out there Metro set up. Other than that we had so much fun!!!
My kids loved the Natural History Museum. We were only there for 2 days and we missed the Air and Space Museum but plan on hitting that one next time. We did the monuments of course and saw the White House.
Bring comfy shoes as we did a lot of walking!
We saw the 3D dinosaur movie at the IMAX theater at the Natural History Mususem and at the American History Museum, on the second floor, there is Dumbo(seriously, from
Disneyland I think?) My 1 year old was trying to jump off me into the exhibit.
My 7 year old had his own disposable camera and loved taking pictures of everything. He got some great shots and has decided to save his allowance for his own camera.
If you are driving and have time, take the 30 minute(or so) drive to Mt Vernon. It was really neat seeing George Washington's house. It is so beautiful.
They are sticklers about no photography on some exhibits: The Star Spangled Banner at the American History Museum(they kicked some guy out because he wouldn't stop taking pictures) and Washington's teeth at Mt. Vernon(an alarm goes off! It wasn't us, it was some kid)
LOL on another note: I am watching this show on the travel channel right now called Mysteries of the Smithsonian. They showed some stuff we missed and now I gotta go back and see it!
Also, now that we have been, my 7 year old keeps beggin us to see the new Night at the Museum sequel. He says cause we've been there so now he has to see the movie!

He also asks when we are going back...We got back two weeks ago...
Have loads of fun!
Don't stay in Silver Spring, Maryland. Thats where we were. I didn't see many nice areas and our hotel was definitly in the ahem...ghetto(I hope nobody on here takes offense to that). Try to stay in D.C. even though its a little more pricy. Most of the hotels I noticed are in walking distance of everything.
Oh and if you wanna see the Ruby Slippers(from the wizard of oz), I believe they are in the Air and Space Museum, although I dont know why they would be there.

I saw them a long time ago when the Smithsonian did a tour and they came to Providence Convention Center.