Peanut-butter/banana sandwiches - yummy, non perishable, really, and pretty sturdy.
Cheese and crackers.
Fruit - oranges fare better in a backpack than apples or bananas.
Cereal/make you own trail mix - dump various ingredients - cereal, nuts, raisins, whatever - in a ziploc bag, it'll self-mix getting tossed around in a backpack.
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