Festival camping don't take anything you'd be too upset at losing/getting ruined.
Take something to tie on to your tent like a flag or bunting if you've got a common tent! It can be hard to find your tent after a long day in the arena.
We generally take a small stove and kettle, and a single pan for frying up bacon and eggs in the morning. Wraps generally travel better than bread, and negate the need for plates. Or porridge pots for breakfast.
It's the only meal we usually cook at festivals as I resent paying thirty quid for two breakfast sandwiches and two coffees! Generally we will eat in the arena for our main meal.
Snacks - just add water noodle pots, jelly babies, biscuits (non chocolate in this weather!), jerky. Tbh I'm not the biggest snacker in the world and none of my preferred snacks are suitable in any event.
Spare toilet roll or bags of tissues as it will inevitably run out.
A female urinal bottle for the tent (males can use it too). Controversial but slogging across a festival field in the middle of the night to the loos is worth sacrificing a bit of dignity for IMO.
Puppy training pads are great to put wet/muddy shoes on. If I'm normal camping I'd just leave them in the porch of the tent but wouldn't at a festival.