Couple of things that people can find good to have... they are available to purchase in the US if you're driving but if you're on site and no car, best to take them with you!
Zip lock bags. Especially good if you're on the Dining Plan because often you can't finish your dessert and popping it in a bag you can save it for later. Also good for wet clothing.
Wet wipes. I've been glad I had these in my back pack many times with my kids!
As others have said ponchos are HUGELY expensive in the parks so getting them here on the cheap is smart. Also sun cream. There was a time when it was cheaper in the US, but now, not so much, especially at the stores on-site!
Something I find useful too is a gym bag. You the ones that kids use for P.E. kits or swimming kits? Firstly, they are good in the parks because they are waterproof for the rides where you will get wet. Secondly, when empty, you can just screw them into a ball and put them in your pocket, then just pull them out to put wet clothing in.
Blister plasters are vital. I know that you are used to DLP but you'll do a LOT more walking at WDW because the parks are much bigger.
