Definitely take some time to walk around the mall. It's quite large, but I'd recommend heading over in the direction of the capitol, or going over to the Lincoln Memorial and Vietnam Memorial area.
There's nothing like strolling around the Mall when you're in a pensive mood. With the White House to the north, the Capitol to the east, and the Supreme Court just beyond that, you are truly in the center of the western world. It's a very powerful feeling.