I always get asked what to snack on during long trips. It can definitely be tough given limited options. Airport food, however, has gotten a lot better. Most places have legit options and the news shops always have protein bars in an emergency. The main focus is to stay hydrated as to not confuse hunger with thirst, then to dive into protein options and quality carbs.
Here's a list of go-tos that are caffeine free, and salt/sugar free. We can't have any dehydration when we're on the go!
#1 - Greek yogurt from the news stand. Pick plain. Forgo the flavored / extra sugar.
#2 - Protein bar. Quest or Thinkthin. Sugars must be below 8g, carbs try for under 15g!
#3 - A meal! Think balance. Naked protein (no sauces or breading), whole grain side, or no side and add a salad!
#4 - Green teas, water, salt free nuts, hard boiled eggs, egg sandwich on whole grain bread, or KIND bars are other adds on to consider. Fruit is also an option.
My advice is to save your eating for when you've arrived/landed. Think quality!