Yes, you are fully allowed to bring your own sandwiches and other "individual portioned" food items into both Walt Disney World and Disneyland for self-consumption. This is a popular way for families to save money on dining costs. The only major restrictions are that the food must not require heating, reheating (no microwave access is provided), processing, or refrigeration, and it must not have a "pungent" odor. You are also prohibited from bringing in glass containers (except for small baby food jars) and loose ice or dry ice; instead, use reusable ice packs to keep your sandwiches cool in a soft-sided cooler. Your bags and coolers will be inspected at the security checkpoint, so simply inform the Cast Member that you have food with you. Once inside, you can enjoy your meal at various picnic spots or outdoor seating areas throughout the parks, provided you do not take up tables in busy table-service restaurants.