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.