According to the Disney Dining Plan brochure a snack includes 1 of the following...
Frozen Ice Cream Novelty, Popsicle or Fruit Bar...Popcorn Scoop (single serving box), SIngle Piece or Whole Fruit, Single serving Grab Bag of Frito-Lay's Chips, 20 oz. Bottle of Coke, Diet Coke, Sprite or 24 oz. Dasani Water, Medium Fountain Soft Drink or Apple Juice.
However, I've read a couple other posts here that occasionally the vendor will accept something different. However, my guess is it will throw off their "drawer" and they'll be coached on it, so I would suspect less and less variations as time goes by, but I'd like to hear of instances where there was something different accepted.
IMO, I'd like to see an individual snack carts/stands sell.