DC area

How many days would you allow to see national mall area & Smithsonian museums?

Depends on what you want to do. You can easily spend most of a day at the Air & Space Museum (see the movies, too). The Museum of American History & Museum of Natural History are also quite popular, too. The National Gallery of Art is a big deal, too, and also right on the Mall. They're all free - and check the Smithsonian website, there are smaller museums that you might like. The Capitol is a better tour than the White House, but you might want do do both. Depending on the weather, the Washington Monument, Lincoln Memorial, Jefferson Memorial & WW II Memorial can be done in a day - if you really like to walk. The FDR, Martin Luther King, and Vietnam Veterans Memorial are also in the Mall area. If you're in DC during Cherry Blossom season, add extra time to tour the Tidal Basin area around the Jefferson Memorial.

The view of DC from Lee House in Arlington Cemetery is memorable, as is the cemetery itself.

The National Zoo (Pandas!) is on the Metro, but not on the Mall (get off at the Cleveland Park Station and return to the Woodley Park - National Zoo Station, that way it's a downhill walk both ways, the Zoo is about halfway between those stations).

The Air Space Museum has a huge annex out by Dulles Airport, too. Haven't been there, but check their website, I think they charge a lot for parking.

Don't try to do too much in one day, and devote as much time as you can.