Hi Cynthia! You'll have snack options galore on the Disney Dining Plan! You're right that the Dining Plan brochure gives only very generic examples of the types of snacks you may purchase with your credits, but you'll see during your vacation that there are actually many varied items available as snacks. The unofficial website, disboards.com, maintains a list of snacks that guests report being able to purchase with their snack plan credits. While it's not an official list, it does give you a great idea of the options available. When you're in the parks or at your resort, just look for the Disney Dining Plan symbol next to the food items at quick-service and snack cart locations. This symbol lets you know that you can use your snack credits for those items.
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Liberty Inn is actually a quick-service restaurant in Epcot with available snack options. (I think you were thinking of the Liberty Tree Tavern, which is a sit-down restaurant in the Magic Kingdom, and you're correct that this table-service spot wouldn't have snack items.)
Some of my favorite Disney snacks that are on the Disney Dining Plan are the candies at Karamell-Kuche in Epcot and a Dole Whip float at Aloha Isle in the Magic Kingdom. You'll also find that your resort quick-service restaurant will have several snack options such as muffins or croissants that could serve as a light breakfast.
Have a magical vacation! Come visit us again at the Moms Panel if you have more questions! We'd love to help!