LOVE this tour!
The Keys To The Kingdom Tour is a great way to have a better understanding of the Magic Kingdom - the planning behind it, the ethos and the day to day workings. You do go behind the scenes to the parade floats and also down to the utilidors which are underneath the park (- it's another world down there!!) during the tour.
The only time you get to sit down is when you stop off for lunch or sit on a ride ( the rides we went on during the tour were the Jungle Cruise and Haunted Mansion but this varies), and it does take over 4 hours so bear in mind that you will be standing and walking most of the time and if it is a hot day you will be wilting at the end of it, but I'd thoroughly recommend it if you are really interested in the workings of the park and the ethos behind it.
(No-one under 16 is permitted to take this tour and you will need admission passes to the park as well).