I'm playing a
Travel Agent tonight....I spend a good bit of time in DC and I always stay in Crystal City (across the Potomac in Virginia) Depending on your price Range....the farther out you go, the less expensive it becomes, but Springfield is as far out as I'd stay--avoid Greenbelt, Maryland (like the plague)
...you can take the Metro Trains, which are safe, straight into the Smithsonian Station (go figure) and be right on the mall at the Museums. They'll love the Air and Space Museum and the Museum of Natural History as well...All the monuments are in the same general area, but it's still a lot of walking....
Everything is pretty much in that centralized area except the Zoo--take the Metro to the Zoo stop and follow the directions from there, you can't miss it
Places I'd stay....depends on your budget..
1. Embassy Suites Old Town Alexandria (directly across the street from Metro stop)
2. Marriott Crystal City or Marriott Crystal Gateway (right next to each other) in Crystal City
3. Last Choice is Embassy Suites Crystal City--simply because it's not convenient to metro.
There's a Hyatt, a Hilton (I think) and another Hotel at Pentagon City that's good to go as well, but I've never stayed there (there's a metro stop at Pentagon City)
Go see the Changing of the Guard at the Tomb of the Unknown Soldier.
Lastly....DO NOT ATTEMPT TO DRIVE IN DC....
you'll thank me later.