The Old Post Office- boasts incredible, expansive views of DC from the Washington Monument all that way to the Capitol Building. I am told that the views are even better than those experienced from the top of the Washington Monument. Access is easy and not crowded at this location and in my opinion well worth the short walk from the mall. If your in town check it out!
View from the top of the old Washington, DC postal office.
The World War II Monument - has in a hidden away service access area a piece of graffiti that is permanently etched into the granite. While this may at first "seem" like a piece of vandalism it actually is not and the history behind it is fascinating. Check out "Kilroy was Here" for more information.