I suggest going to Tyson's Corner Virginia if you want to use your 5 hours. If you can rent a car, you may be able to do a round trip to the White House and back in 2 to 3 hours.
also crucially depends on whether this is rush hour or not. check to see with your Google maps or Uber time estimates. Also, best to be focused on which attractions you're interested in seeing
It can be around an hour and a half to get into downtown DC from Dulles, round trip would be 3 hours. Give yourself an hour flex time to get back into the terminal and through security and you could be at a total time of 4 hours with an hour to see the sights! As long as you don't have to wait for a taxi for too long coming or going and you may be able to do it and have it worth your while and cost. Or were you referring to a different downtown?
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