If you want to go all out, get a little propane Coleman stove, and some Backpacker's Pantry freeze-dried food. Other than that, I second mushrooms, E, and water.
when i went three years ago i just cooked pasta in my wisperlight backpacking stove, but i mostly just ate junk food the whole time. pop tarts, chips, little Debbie products, ect.
make sure you hit up the little gas station for breakfast sandwiches, they are the shit. plus they have a gigantic special one that you win a t-shirt if you can finish it
canned foods that you can eat as is or heat them over a grill/fire (soups, spaghetti o's, corn, etc)
hot dogs/brats/burgers
potatoes
nutrigrain bars/chewy bars/any kind of cereal bar/etc
trail mix/nuts
chips
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.
Bread, Peanut Butter, Apples, Kiwis, Oranges, Pineapple, Marshmallows, Skewers and Veggies/fruit (or meat if you eat meat), JiffyPop, and don't forget water. Bring lots and lots of water
fresh veggies and fruits are good out of refrigeration for several days. peanut butter, tuna and bread. always good. canned food. and sausages are cool if you have a cooler.