We just got back yesterday, so let me think....
-extra Ziploc and plastic baggies for all those wet clothes/lotions/etc that you wouldn't want in your suitcase;
-bug spray! The bugs weren't terrible, but if they tend to "like" you (like they do me), I was getting a little itchy in the evenings waiting for fireworks
-extra walking shoes! Our shoes got SOAKED at MGM one night in the rain, and mine were too wet to wear the rest of the week. Thankfully I had a backup. Also a pair of flip flops to wear to the pool/foodcourt/around the hotel.
- Snacks... we made PBJs to take to the parks to snack on, more filling than chips or whatever.
-Crystal light or Koolaid packs - yes, the Florida water is gross but this makes it more palatable. And on ice, the water itself really isn't so bad! We did not buy one bottle the whole week, I cannot imagine how expensive it would have been to buy all the water we drank
- A paperback book to read in lines, or a book of crosswords etc - we brought a small crossword puzzle book and worked these together, SO great at making waits bearable!
- Sturdy ponchos: I saw a lot of people in the cheap Walmart ones (you know, the kind that look thinner than a garbage bag), and those DO NOT hold up in any rain that is more than a light drizzle! We were so happy to have thick, sturdy ponchos!
-Misting fan: We got ours at Walmart for less than $4 and they were selling them for about $18 at Disney.
-Put your cell phone in a ziploc baggie on days it might rain, or if you are going on a water ride - I thought mine was safe, but it got zapped in a rainstorm
- All your various chargers, etc: we brought my laptop charger, the rechargeable battery charger, iPod charger, etc...
And definitely, TAKE A MIDDAY BREAK! Even without children, we tried to leave the park around 2 or so and come back to the hotel for a swim, nap, and shower - we would go back around 6 or 7 and feel rejuvenated. Otherwise the heat and crowds would have just been overwhelming.
I'm sure I'll think of more! Blanking out right now!