Found this place while wandering aimlessly around Rosslyn (a burough in Arlington). Built at the top of the tallest building in Arlington (I assume), you can see most of the DC Metro area. There are historical panels around the interior of a large glass hallway facing outward in all directions. You can see things like the water tower over CIA's Headquarters at Langley, VA, the capitol and downtown and other historical points.
There's even a little coffee shop built inside. I'd say it's a bit pricey ($22/adult) just to go up and look around, even if the decor is nice and you don't have to go outside if you don't want to stand out on the upper deck open balcony. Historical and a great view, it's still worth going to see.