Is 6-hour layover enough for a short sightseeing?

Assume you land on time.  You're looking at an hour landing until you are in DC.  You need to be back at DCA at least an hour before your flight (probably a little more depending on time of day and what type of security pass you may or may not have.)  So that leaves you 3-4 hours of free time in DC around the Smithsonian.  Assuming you don't have bags which may cause problems with security there, that is a lot of time.  I would go for it and just not push the return.  Decide 1 or 2 places I wanted to see and go for it.

not if you have to go thru customs ..........................

yes, if you stay close to the airport.  I have visited the surrounds of Philadelphia during a 5 and 1/2 hour layover.

i agree to some extent with DEBS, but it all depends what time of day you are landing in and what would be the traffic along the routes you wish to visit.

You'll have time to take the Metro (subway) to see the Smithsonian and Washington monument but not have a lot of time there. 

Be willing to spend on taxis and allow more than enough time to get back to the airport. Rush hour traffic can be paralyzing in Washington DC.

No, I don't think so.  It would be risky.

Stay in the airport and use the time to get to you next terminal.

Visit the space museum which is only 3 miles away. I would definitely not go to Washington Dc you don't have enough time.   

No. Stay where you belong.

Please report the commenter on this answer for using unwarranted hostility and defamatory slur. Typical butthurt democrat.