If you are going into any Smithsonian museums you'll have to go through security, so be prepared for that. Don't try the Capitol building. I don't know what optins you are considering. I can think of two: Metro and taxi. Metro is convenient from the airport, but you could end up walking some distance to gt to museums, monuments, etc. Not unduly long walks, but if you're carrying your carry-on bags with you, it could get tiring. You should be more specific as to what you want to do. Will this be your only chane to see DC's attractions? If you don't want to go into museums and just want to see attractins from the outside, you could look up where to catch the Tourmobile, which makes a wide circuit along the Mall, etc. with a guided narrative. Remember that you can end up having to take multiple escalator or elevator rides to get into or out of a Metro station.

If you have 6 hours total., and need to be back at the airport 1 1/2 hours before yoru next flight, going all the way to Union Station to check bags and then touring, might not leave you a lot of time.