Personally I love the Metro. I go down to DC a few times a year for conferences, and LOVE THE METRO.
Although I don't know where the campgrounds are, you may be out of the METRO jurisdiction.
I personally think the tour busses, tourmobile, duck tours things are overpriced, and you can get to the points cheaper.
As for Smithsonian, there are about what 5-10Smithsonian musuems... so you have a lot to choose from.. Many think its just one musuem...
There is Air and Space, Natural History, 20th Century History, Potrait Gallery (this was closed for a while), Holocaust Musuem, ARts and Industries, Modern Art, and a few others off the top of my head... all the other stuff Arlington Cemetary, Washington Monument, Lincoln Memorial, Vietnam Wall,etc....
The METRO is very very inexpensive as well.