The Air and Space Annex closes at 5:30pm. Just went this past weekend and it is worth the trip. I don't think the same can be said about the one on the Mall, but I have been there about 4 or 5 times in as many years. I think it is good for an hour or two tops. Natural History and American History musuem are IMHO much better. I believe that American History has opened a major transportation exhibit in the past year or so that is worth checking out.
And all around downtown DC, Foggy Bottom, Georgetown, Adams-Morgan, DuPont Circle are tons of bars and restaraunts to choose from
As for getting into the city, it all depends on where you are staying. Cheapest would be to take MARC or Amtrak from the BWI
rail station into Union Station. Then take the Metro or cab it from there.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia