My daughter lives in DC. Thankfully she will be moving to Alexandria in FEB. Last time we went to visit her she took us to see the National Cathedral. It is truly amazing. You can take an elevator up the tower and look out over the city. One of the coolest sights was looking out over Reagan National. You see the planes banking into the runway flight path to land.
Arlington Cemetery is a must too.
That just proves what I am always saying about DC. There is always something to learn about the city. I've been to the National Cathedral a number of times and did not know there was a trip to the top. Is that for the general public?