Hi Chasity, welcome to planDisney!
I am pleased to inform you that per
property rules you can
bring a cooler with food and/or drinks into the parks as long as it is not larger than 24" long x 15" wide x 18" high. Certain items are not allowed, such as glass containers and alcoholic beverages, but otherwise, you are welcome to fill your backpack with items you will need throughout the day. You'll want to stay away from loose ice so I recommend packing reusable ice packs or sealed bags to put ice in to use as an ice pack.
When I arrive at a Disney Resort for my vacation, one of the first things I do is place an Instacart order containing snacks and plenty of water bottles. I load up the refrigerator in my room with the water bottles so they are ready to go in the morning when I'm ready to head out to the parks. If I'm lucky, the refrigerator will have a small freezer included which works out well as I can pack a couple of frozen water bottles to double as an ice pack. I prefer to use a backpack cooler and it seems to work well for our family.
As you probably know, it is very important to stay hydrated at Walt Disney World so I'd also like to mention that you can request complimentary cups of ice water at quick-service locations if needed.
Have a magical vacation!
Diane