A tip for seeing how busy the Magic Kingdom is
A quick way to tell how busy the Magic Kingdom is right before you arrive is to see how many trams are being used. On your way to the Magic Kingdom, have a look to your left if you're on a WDW bus at the rearmost parking lot and see how many trams are still there. If you're driving, it's a bit more difficult but they're on your right.
The Magic Kingdom has 7 parking lot trams and they're all lined up in perfect rows until needed. By seeing how many trams aren't being used, you can gauge how busy the park is at the moment.
Sometimes very early on slow days, all seven are there and when EVERYONE is heading to the magic Kingdom, you can tell because there will only be one or two there. I've always seen at least one there so I assume there must be a heavier load of people leaving the park than entering.
I know this is a bit weird but we pass by that lot a LOT heading to the Magic Kingdom resorts to deliver Lockers and I always glance over at the trams out of curiosity.