National Gallery can indeed take all day, but of course one benefit of all the Smithsonian museums is free admission. No need to feel like you need to get your money's worth out of it. Many of them will produce overload if you try to see the whole place at one shot, but fortunately one doesn't have to.

Buying the SmarTrip cards is a good idea - since now the cards are only $2.00 (they used to be $5 each), and there is a $1.00 surcharge for rail trips on a paper ticket. One round trip and you've paid for the card in the surcharges avoided.