Yes, you are absolutely allowed to bring your own snacks and non-alcoholic beverages into Walt Disney World and Disneyland. This is a popular way for families to save money on "food and beverage" costs during a long park day. You can bring items like granola bars, fruit, crackers, and even small sandwiches, provided they do not require heating and are not stored in glass containers (with the exception of small baby food jars). You may also bring a cooler, but it must be smaller than 24" long x 15" wide x 18" high, and loose ice or dry ice is strictly prohibited; you must use reusable ice packs instead. During the security screening, you must declare your food items. While you can enjoy your snacks in most public areas and queues, you are generally not permitted to eat your outside food inside the "Table-Service" restaurants. Bringing your own refillable water bottle is also highly encouraged, as "Cup of Water" is free at all Quick-Service locations, allowing you to stay hydrated without paying $5 per bottle of Dasani.