4 day festival

I go for 5 days every year, I take some protein shake cartons/bars for breakfast, some snacks like babybels (they keep ok out of the fridge) or salted popcorn whilst drinking and then usually just have stuff from the stalls early evening and make as best a choice I can, I try to stick with falafel/chicken, you'll get a lot of walking/dancing in so I wouldn't worry too much about the calories, it's not like it's something you're doing every week.

I also tend to stick with soft drinks/water when in the arena and just have alcohol on the campsite, which saves me a bunch of calories and cost, as I will usually have spirits and diet soda rather than beer.

Even if you don't have a cooler, freeze some bottles of water before you go, then you can use them as Ice Blocks and have some cold water available as it thaws out.

If you have boiling hot water, you can make oatmeal and stick some peanut butter/nutella in it for a hearty breakfast! Or bring a tea kettle if you have access to a plug at your campsite.

Fruit/veggies that keeps well: apples, oranges, tomatoes, peppers, avocado. When I am at festivals the last thing, I want is something greasy or salty if it is hot out.
Granola, dried fruit. If you aren't a fan of drinking pure water you can bring country time lemonade packets and add them to water to make lemonade

"Snot Poodle" (a.k.a. Pot Noodle)

Oat So Simple porridge pots.

rye bread and canned spreads / non perishable spreads