We keep granola bars, cheese sticks and yogurt tubes on hand for on the go snacks. My kids also like to snack on dry breakfast cereal and we've been making our own snack mix which consists of whatever we have - cereal, pretzels, raisins, yogurt covered raisins, choc chips, goldfish, mini marshmallows, etc.
A sit down snack at the table would be cut up apples, carrots with PB to dip, crackers with cream cheese or ants on a log.