Tips for festival grub that lasts?

[irrelevant]

Rice cakes, flapjacks, nuts & dried fruit for sustaining snacks.
If you have a gas stove, IMO rice or oat rice hands down is the best to bring. Accompany with some pesto, tapenade or similar, sprinkle seeds and sliced almonds on top, and drizzle lemon or lime juice over. Healthy, easy, yummy.
You can make several portions at once, and just keep overnight in a sealed container.