If they can eat peanut butter, I like the packaged peanut butter and cracker sandwiches (they come 4 or 6 in a package). They're not the healthiest things on earth, but this is a special occasion and they hold up very well in the heat.
So it's not the healthiest thing but...I buy those 8 packs of the little boxes of kiddie cereal (frosted flakes, froot loops, etc). My DD doesn't usually get the sugary cereal, so it's a vacation treat. I keep telling myself "at least it's fortified with vitamins and minerals!!!"
Quaker Oatmeal Squares in Cinnamon (in ziplocks)
Annie's Cheddar Bunnies
Fruit Snacks
Organic Lollipops (for waiting in line)
Boxes of Raisins (it was yogurt raisins, but I actually looked at the nutrition labels on the ones I gave DD at breakfast, and those suckers have 18G of sugar per serving. Holy dorsal fins Batman! We won't be buying those anymore!)
Peanut Butter Crackers
We'll also grab some grapes and apples on the first day and keep those nice and cool in the fridge.
We do a lot of Odwalla bars as snacks. They are filled with protein (to avoid the blood sugar spike and crash!), don't melt, don't crumble/get smooshed, and the kids love them. We keep a stash in our diaper bag at all times for when our kids won't eat a regular meal.