Chewy trail mix bars. They are cheap (Nature Valley or the house brand), and in the sun they get gooey enough so that you can take a small bite and not have crumbs everywhere!
The regular granola bars are excellent, too. I do perfer Nature Valley, but watch-out- they sometimes use hidden trans fats in a few select bars.

Also, I fully endorse the powder mixes, single servings. I love them, and asking for ice and/or water is easy at backwoods stores.