Bagels or English muffins don't squeeze as much as bread slices, so some people prefer them.
Fruit is good; bananas are usually OK for half a day if they're not too ripe, oranges, apples, and pears travel well and are satisfying.
Cheese is fine (at least up to 90F) without refrigeration for a day or two. Kraft's zip-lock packaging keeps it neat.
Pop-tarts can be eaten cold, although they might crumble they're still edible if you can wash them down.