I love The Old Post Office in DC. I went here for the first time not too long ago and you can read about that visit here. It's easy to access from the mall, free and provides a wonderful view of the city and surrounding areas. What's not to love about that. I took Mom and Stephanie here when they visited and we went on one of the most clear days they've had in a long time which meant we could see things off in the distance that aren't normally visible. We were even able to catch a small glimpse of the temple which was neat.
Love the views of the Capitol. A bit of a challenge to focus my camera in a way that doesn't show the bars (you can still slightly see them).
The White House hiding behind the Treasury.
The Lincoln Memorial.
Mom and I at the top.
The National Cathedral with construction (thanks a lot earthquake).
I love all of the cute rooftop terraces in the area.