One of my favorite snacks to bring on board is a ziploc of Gummi Bears. They're sweet and don't melt (like chocolate) or break (like crackers). Unlike other dry snacks (like trail mix or granola) which are typically salty and require water, Gummi Bears don't make me overly thirsty. They can be squeezed into any luggage no matter how full, and even if the ziploc breaks, there's no sticky/crumbly mess to clean up. Also can be easily gifted to parents of unruly children as the situation requires.

A several-pound bag at Costco is $5.99, which is just slightly more than you will pay for a couple ounces at the airport. Don't buy snacks at the airport!