Our snacks have to be nut free. Off the top of my head, I've provided the following:
goldfish crackers, pretzels, teddy grahams, graham crackers, etc.
squeezable yogurt/squeezable applesauce
blueberry mini-muffins (store bought, usually Costco)
snack size bags of Pirate's Booty
I try to mix it up so the kids don't get bored. Our preschool teacher(s) gave us the heads up that Nutri-grain bars don't go over very well.