Hi, Mauryne!
We've brought just about any kind of snack from outside the park into Walt Disney World. That has saved us dozens of times when somebody suddenly decides he's hungry or somebody just needs a little pick-me-up to hold her over until the next meal.
We each carry a resealable, wide-mouthed insulated cup, too, to save us from having to interrupt our tour of a park because someone is thirsty. This type of cup will allow you to fill it up with a drink from a Quick-Service restaurant, which may contain lots of refreshing ice. Don't want to leave that behind because your refillable cup's opening is too small. :-)
Our favorites include fruit snacks, lollipops, popped popcorn, peanut butter or cheese crackers, trail mix, breakfast mix, protein bars, granola bars and pretzel sticks and cheddar cheese crackers. I also bring zip-shut baggies to help contain contents of opened packages as well as for portable leftovers from meals (carrot sticks, grapes, cookies or brownies.)
Take a look at this link from Frequently Asked Questions regarding bringing in
outside food to Walt Disney World parks and Resort Hotels. I'm sure you'll find this really helpful!
Send more questions as you think of them, Mauryne; I love to help!