Canned tuna is an option, but another good one could be canned chicken. You could bring both and buy croissants and have both chicken salad and tuna salad sandwiches. You could make a nice chicken salad with a dried fruit (craisin/raisin) and/or jarred mandarin oranges and a nut (sliced almonds) with a vinaigrette (olive oil/red wine vinegar) or mayo (I'd buy the very small jar so you could use it up as a single use or maybe packets) and salt/pepper. A simpler, spicier chicken curry sandwich would use the canned chicken with curry powder, mayo, and chili oil (and a sliced nut if you like nuts). Veggie or regular chips and shelf stable fruit (apples, bananas, oranges) and you have a full meal.
And to add on to the chip idea, to make it palatable cold, I'd suggest a mango-corn-black bean salsa with pitas or tortilla chips...cold beans normally need a freshness to taste good and the following is completely shelf-stable (I eliminated the cilantro that the recipe called for b/c I think that wouldn't work as shelf-stable)...buy the mango and lime whole...
What You'll Need:
1 (15-ounce) can black beans, rinsed and drained
1 (15-ounce) can whole kernel corn with peppers, drained
1 mango, peeled, seeded and diced
1/4 cup chopped onion
2 tablespoons fresh lime juice
1 teaspoon garlic powder
1/2 teaspoon salt
1/4 teaspoon ground cumin
And a fun fruit side/dessert is to make shelf-stable lighter ambrosia...a can of drained diced pineapple, a can of drained mandarin oranges and a jar of drained maraschino cherries, tossed with some mini marshmallows and dried coconut (mint is optional) and you have a fun, very kid-friendly side

. You would have to eat this in one sitting, so choose your fruit jar sizes appropriately...