Ooh I'm going to a festival next week I wonder if it's the same one?!
Veggie chilli - tin of beans, tin of red kidney beans, tin of tomatoes, pinch of chilli powder. Nice and easy, no fridge needed.
Dried pasta n sauce meals (made with water omit milk or use long life)
Pouch of micro rice made into cold rice salad with sweetcorn, peas, fresh chopped veg etc.
Pasta with pesto or jar of sauce.
And obligatory girl guide puddings;
Martian pudding - angel delight, chopped mars bars hidden underneath
Magic mushrooms(!) - angel delight, slice of swiss roll, half a tinned peach on top
Those tinned meals you can get in supermarkets too - one of mince in gravy, with a tin of mixed veg stirred in and another of tinned new potatoes makes a respectable meal.
Variations of eggs, like fried egg butty, frittata (tinned potatoes sliced and an onion, maybe some tinned ham), scrambled eggs etc.
I take a coolbox, ice packs and fairly sturdy fruit and salad veg - peppers, cucumber, celery, red onion, carrots, apples. I can't resist taking strawberries and cherries but eat them up first, they don't last. I take crackers and hard cheeses too (hard cheese lasts a few days in coolbox ok) and longlife sandwich paste, tinned ham and peanut butter/marmite, and we have a "spread/make your own" type breakfast and lunch with those things. A watermelon will do reasonably well a couple of days if you only slice as needed, too!