First thoughts: You might build your plans around how you will get to and from the two Air & Space locations.

If you will be renting a car, it's not as much of an issue, but if you are not planning to rent a car, how you will get to and from Udvar-Hazy should be considered first.

Since you are flying into IAD, you have the option of visiting U-H first (before going into the city itself). Public transit and taxi options (and parking information also) are listed here:

http://www.nasm.si.edu/visit/transpo...n/airports.cfm