There really is no complete listing since it can vary. Disney uses examples like bottled beverages, popcorn, ice cream, fresh fruit, etc as what you can use as a snack option in their marketing materials. However, it is usually the rule that if something is under $4 it can be used as a snack. Just look for the DDP logo as you tour the parks and that will let you know what you can use your snack credits for.
Just to clarify, the general rule now is any single-serving thing under $4. Sometime in 2008 they started applying the "single seving" provision more specifically. I used to use snack credits on a large bag of candy or snacks in the hotel gift shops, but they've done away with that -- even if it's under $4, if it clearly isn't intended as a single serving thing, it's a no go.
But in the parks, pretty much anything under $4 at a counter-service restaurant, or a cart or other location, will qualify. For me, the Dole Whip float is my favorite use of a snack credit.
Credit disneygirl76 for this very extensive snack credit link:
http://www.associatedcontent.com/article/1539859/get_the_most_out_of_your_disney_dining.html?cat=16
Wow, looks like all the Frenchy pastries are included, even the Ham and Cheese Croissant. That's a meal in itself. A cup of soup is also included from Sunshine Seasons. Cool.
Much better than 2 years ago when they cut it back to ice cream, fruit, chips, popcorn, and drinks.
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