Since the parks will most likely close on Christmas/NYE due to the high attendance (Disney, not sure about Universal), if you're not staying on property (i.e. you don't have a meal plan), what about lunch and/or dinner reservations? That will give you entrance into that specific park and you won't have to wait in a long line for food.
Purchase the photo pass, if you weren't already thinking of that.
Disney gift cards so they can maybe get that special souvenir they wouldn't normally get - Dooney & Bourke bag, the certain bracelet, make your own light saber, etc.
You can rent a party boat to see the Magic Kingdom fireworks from the bay.
Or even one of the dessert parties-in the parks or the pirate cruise one where you can meet Capt Hook and Smee.
Depending when you'll be there, get dinner reservations so your guaranteed seating at the Candlelight service at Epcot.
You can also ship things to your hotel (both Disney and off-site). If you're the only ones flying, maybe ship gifts you'd normally give. No one else should (hopefully) have space issues on the drive back.