Many people go into the parks with nothing but their tickets and have a grand time. And truly, just about anything you could possibly need in the parks you can purchase in the parks including sunscreen, rain ponchos, Tylenol (usually behind the counter, get help from a cast member), baby food and formula (at the baby care centers), and extra children's undies (long story, don't ask).
I, however, am a "be prepared" kind of gal, so I usually bring a big bag of stuff into the parks with me. You can see my list of things I bring into the parks
here. Disney Moms
Kara and
Laura have made similar lists, just click on their highlighted names to find them.
Of the things on these lists, the one thing I would be most lost without is my camera. I do use the PhotoPass photographers also, but I value my candid vacation photos so much that the camera is the one thing that I would find it really sad not to have with me.