I have quite a few must haves myself.....
*Lots and lots of cheap ponchos from Wal-Mart and the dollar stores. You can use them and throw them away. You can also share.
*A hard folder to put park maps and any paper pieces I collect for scrapbooking.
*Too much film....better to have too much than not enough and have to pay Disney by the roll.
*Mosquito Spray
*Lots of extra socks and undies. Gotta have enough so that if we go back to the hotel and take off our shoes, we have a fresh pair to put on.
*I print up a small card on cardstock paper (and laminate it) that has all of our reservation numbers on it as well as our resort phone number.
*Labels from my laser printer that have our name, cell phone number, resort name and resort number to label film, cameras, camcorder tapes, etc. Just in case we drop the film somewhere, hopefully a CM would return it to us.
*Travel Size Toiletries - We bring all of our shampoo, conditioner, soap, lotion, shower caps, deodorant, tooth paste, tooth brushes, and everything else in small disposable sizes so that we can just toss everything at the end of the trip. This gives us room for souvenirs and the little things we pick up from here and there.