Do not underestimate how much you will spend on food. It sounds silly, but once you get down there (and you're at the parks themselves having a great time) you will be surprised at all the little snacks and things (especially beverages)that add up. The "hey, I'm on vacation attitude" is what got us. For us, a family of 4 (31, 28, 5, 4) we ended up spending almost $100 a day (sit down meal) just for lunch.
Also consider locker rental, towel rental at the water parks if you're going to do this, stroller rental (which is definately not cheap), fridge rental, pictures with the characters and/or your family you might want to buy, two tons of quarters for your laundry, room service tips, and of course, any unmentionables you may forget.
We always budget at least $500 miscellaneous expenses just for this purpose.