The Arlington National Cemetery, in Arlington, VA are the grounds that honors those who have served our country. This is still a very active cemetery that holds numerous funerals daily, so when visiting, please be respectful of these hallowed grounds and the other visitors (Silence your cell phones!!!).
The grounds are huge, but you are able to walk through the majority of the cemetery. A better way to get around and see everything though is to take the tour bus for $8.95/person, which will make 3 stops along the way. The buses keep coming every 20 minutes, so you can stay at any spot as long as you want and venture around as much as you want. This is especially helpful on extremely hot days.
The bus will stop at the Kennedy Grave site, the Tomb of the Unknown Soldier and the Arlington House. All are definitely worth your time to visit, however the Tomb of the Unknown Soldier is definitely the best to see due to the Changing of the Guard. Every 30 minutes in the summer (60 minutes in the winter), you can watch the "show", which is an intense display of military training as the soldiers change positions. Words cannot describe the impressiveness of this and it should definitely be seen and appreciated by all Americans.
When leaving the cemetery, also keep in mind you are very close to the Iwo Jima Memorial Statue, as well as the Air Force Memorial statue, both in Arlington, VA. They are very impressive and should be visited when in the area.