If you are going to spend the whole day out (from dark to dark) I assume you will be having a morning meal first.
I do just that on occasion... I pack 3 apples, 2-3 frozen bagels (they aren't very big and thaw on stand) a chunk of cheese and/or some lunchmeat or jerky and quart of water. Maybe a candy bar or two..depending on the size of my breakfast.......