If you haven't been to Arlington National Cemetery, it's something you should put on your to-do list. The property was confiscated by the United States during the Civil War when Robert E. Lee (the owner) didn't show up in person to pay his property taxes. (I think he was otherwise occupied)
Since then, Arlington National Cemetery has become one of the most hallowed places in the United States. When you visit, the tone is much more somber and subdued than even the other war memorials around the DC area.
The Tomb of the Unknowns is probably the most famous area, but there is much more to Arlington. Kennedy's gravesite, with the eternal flame is there, and there is also a plethora of monuments and memorials to battles and other events. Arlington National Cemetery captures a truly special note of the history of the United States.
However, the Tomb of the Unknowns is famous for a reason. It is guarded 24 hours a day, every day, all year, no matter what. During the fall and winters the guard is changed every hour. In the spring and summer, they change the guard every half hour. From the official website:
Here's the full ceremony: