I had a 6:20am arrival into Dulles from LAX back in November with an international flight leaving around 11-12 hours later. Found out I couldn't check-in until 2:30pm so was stuck with my case and laptop bag which meant I couldn't really pop into DC without dragging both with me.
Had I known this in advance, I would have hired a car for the day, even if it were only to store my belongings in whilst I took the Washington Flyer / Metro in the city.

Landside at IAD is really boring. Few seats, couple of shops selling newspapers etc and one or two food places. A lot nicer airside as it's more modern, more shops, lounges etc.