One of the most moving of my visits in the Washington area was to the memorial for those died in the terrorist attack on the Pentagon on 9/11. It is a very simple - but very moving memorial. It can be accessed by foot from the Pentagon Fashion Centre (walk westwards from the Macy's northern entrance - cross the road on the pedestrian crossing - and follow the signs to the Pentagon Memorial via a tunnel under the roads). Alternatively you can walk from the Pentagon metro station. Further details are available here.