Ok, I'll go beyond the obvious of clothes and toiletries. Here is my list, no particular order.
1 - Electronic equipment
(Camera, Cell Phone, Laptop (to offload the pics), Palm, batteries, photocard, Chargers, power strip)
2 - Itinerary
(ADR and Confirmation #, Park Opening/Closing/EMH, Special events)
3 - Pre-Addressed Address Labels and Stamps
(will purchase Disney postcards and send to cousins, nephews and even my own DD)
4 - Light up Stuff
(Glo necklaces, Light-up toys, etc -- much cheaper than buying in the park and we have many at home already)
5 - LGMH
(and other decorations for room, stroller and backpack)
6 - ID Cards and Coupons
(DVC card, Photo ID, Disney VISA and Rewards Card, AAA card, Health Insurance card, any coupons for local stores I may visit)
7 - Over the Door Roll-up Bag
(has lots of compartments to put toiletries or cards or whatever in and leaves more room on the countertop ... also keeps things out of my DD3's reach that she shouldn't have)
8 - Small "appliances"
(Nightlight, pull-down showerhead --makes getting DD's shower a LOT easier when you can pull it down to rinse, also it's a massager one that is great for sore shoulders/back, small flashlight)
9 - Ziploc Bags
(all sizes -- for snacks, wet clothes, protect papers)
10 - Entertainment
(trivia cards, books, autograph book, coloring book/crayons, notepad)