Lidl do nice tuna salads in foil - 3 different kinds, and you even get a (very) small plastic fork with them.
Porridge with uht milk and squeezy honey for breakfast, or the pancake mix in a carton which you just add water to and shake. Remember the frying pan and oil.
Nuts and dried fruit to snack on.
I told ds2 the other week you can buy dehydrated water, and if you mix water with it you double the amount of water. He didn't believe me - no idea why 😄
Crackers or oatcakes with Nutella.
Proper salted butter is usually ok for a few days and is nice just stirred into cooked pasta.
If your bread goes stale, you can still make fried bread with it, or eggy bread. Nice with baked beans. Or fry tinned potatoes.
Dried stuff like muesli, tea bags etc, we normally decant into reasealable bags. Instant hot chocolate is also good.
Hope you have a lovely time.