canned foods that you can eat as is or heat them over a grill/fire (soups, spaghetti o's, corn, etc)
nutrigrain bars/chewy bars/any kind of cereal bar/etc
Go to a grocery store and find stuff that won't go bad. If you want food that would need to be kept cold, make sure you don't have too much at a time as you will need to change out ice often. festivals usually charge like $5 for a bag of ice that is about half the size of a normal gas station bag... so it can get expensive keeping food from going bad. my tip is to bring your meats and stuff with lots of ice packed in your cooler and eat the food that will go bad first and then stick to the non perishable shit.