Bring lots and lots of ziplocs.
Bring extra pacifiers. They do sell them at the baby care center, but they're about $6 each. Not a lot when you're stuck, but why be stuck?
Bring warm clothes. I know September is warm, but if you get caught with a cold day, you'll pay a fortune in the parks for warm stuff, if you can find it that small. One thing we've done when stuck is to buy a hooded towel that acts like a blanket over the baby. Sleepy looked real cute with the hood on, too, like a little Pooh riding around in his stroller. Well, Tigger too, cuz we got caught with two cold days...
And if you don't know about the Baby Center, find it. In MK it's by the Crystal Palace. It's a huge, clean facility for changing diapers, warming bottles, feeding in a quiet, private place...anything baby. They sell supplies (up to pull ups) and the staff couldn't be more accommodating.
Have fun!