Travel/trial sized items! My pediatrician always has a box of freebies at his office, and the things that came in handy for me were trial sized liquid detergent for laundry and trial sized packets of diaper rash creme and baby lotion/soap/shampoo.
I also now travel with just a sample sized vial of perfume - even though it's not my everyday perfume (which doesn't come in samples anymore). Just easier than lugging the larger bottle in my cosmetics case.
For the kids, keep a lookout for Disney themed t-shirts, sweatshirts/sweaters/hoodies (since it will be December), and bathing suits at the end of the season sales (Target, DIsney Stores). I was in a panic last December thinking about what bathing suits I'd bring for my growing girls. December weather can range from the 30's/40's to the 80's in Florida.
And after Halloween, buy some glow sticks or light up toys. We brought these light up blinky wands that the girls loved. We also had glow bracelets on the strollers.
Disney in December is great! Have fun planning!