This is an absolute must see. The changing of the guard is a pretty quick ritual but the work/timing/accuracy/precision is mind boggling.
We purchased the train ride inside the visitors center and they time the 3 drop-offs perfectly. They stop at JFK's eternal flame and give you 15 min. Then on to the Tomb of the Unknown Solider just in time for the changing of the Guard then on to the home of the Lee's. You can actually stay at any given spot for as long as you like and catch another bus but the alotted time frame worked great for us. The train tour was 8.00 per person.