You can check out the National Air and Space Museum online at:
Currently, the museum is undergoing a year-long renovations process that has closed some exhibits. One of my favorites, the space hall, is closed. Two other areas are scheduled to open around this time of the month, but you'll have to call them up to see if they're actually open.
Since you're coming into DCA, you can take the blue line from the National Airport station directly to the Smithsonian station. Come up the escalator from the station, and head down the Mall towards the Capitol building. Walk for about four blocks, passing the Smithsonian Castle (big reddish-colored building) and the Hirshhorn Museum (whitish-grey circular building) on your right. The Air and Space Museum will be just past that, on the right. You can't miss it.
Also, be sure not to miss the historical room at National Airport...it's a bit off the beaten path, and it may or may not still be open, but it's in the Main Terminal (which is also currently undergoing renovations). Enter the main terminal from National Hall (terminals B & C), and head left into the main section of the terminal. Immediately to your left is a staircase; head to the top, where the room is. Inside, you'll find the history of the National Airport site, artifacts from airlines that have served DCA over the years, and photos galore. Be sure to pick up a free National Airport/Abingdon Plantation poster, if they have any left.