When my kids were little, I used to buy those variety packs of individual Ritz Bits, Chips Ahoy, etc. They would come in a 9 pack, with three of each snack.
I also brought the packages of crackers, like peanut butter on cheese, or cheese on wheat.
Granola bars
pop tarts
Nutrigrain bars
fruit snacks (only on vacation...I did not like giving my kids these very often!)
raisins
dried fruit and nut mixes (I would buy from the bulk dept. at the grocery store. My kids always liked the banana chips the best!)
ETA: those Hand-Snacks things....like the crackers with the spreadable cheese, or the pretzels you dip in the cheese...very portable!