Yes, you are generally allowed to bring outside food and snacks, including sandwiches, into Universal's Islands of Adventure, provided they meet specific safety and size guidelines. Universal Orlando Resort allows guests to bring small snacks and items that do not require heating. You can pack sandwiches in a soft-sided cooler or a backpack, which is a great way to save money given the high cost of theme park dining. However, you must avoid bringing hard-sided coolers, glass containers, or any outside alcohol, as these are strictly prohibited and will be confiscated at the security checkpoint. Additionally, the food must be for personal consumption and not in large quantities that suggest catering. While you can carry your sandwiches around the park, keep in mind that for many of the more intense thrill rides, such as VelociCoaster or Hulk, you will be required to store your bags in a locker. Bringing your own lunch is a savvy move, especially if you find a quiet spot near the waterfront in Port of Entry or Seuss Landing to enjoy your meal away from the bustling crowds.