Fruit: bananas, Raisins and apple chips.
For nuts go with the tastier ones like macadamias or pistachios.
Cheese sticks are a god send. Throw an ice pack in their lunchbox so they don't get funky.
Strips of grilled chicken keep pretty well in a sandwich bag in a lunchbox with an icepack.
Popcorn is cheap, non-perishable and you can buy the gigantic bags at costco with just some oil and salt as ingredients. Split into snack bags.
Make protein pudding with greek yogurt and honey throw it in a small tupperware container. My kids love that.
Fair warning, they will just trade their awesome healthy snacks for a corn syrup pooding pack and a fruit rollup.