Hi Brian!
On the dining plan a snack is considered most single serving sized items under $4.00. Some examples are bottled water, soft pretzels, Mickey ice cream treats, a single serving of popcorn, small bags of chips, muffins, root beer floats, single pieces of fresh fruit and much, much more! For an extensive list of snack items confirmed by Disney guests visit
www.disboards.com and go to the "Disney Restaurants" forum. Once here there is a sub-forum labeled "Disney Dining Plan" and within this board there is a thread containing a list of snack items.
Another thing to keep in mind is that all items that are considered snacks under the dining plan have a little icon next to them on the menus. By looking for that little icon, you will know what counts as a snack.
It may be worthwhile for you to upgrade to the basic dining plan if you plan to eat at any table service or "sit down" restaurants - especially if any of those are signature dining establishments. I would recommend taking at look at your touring plan and where you would like to eat. You can view menus at
www.allears.net by clicking on "Dining" at the top of the page and then "The Menus" on the left side. By getting an approximate idea of what your sit down meals (if any) might cost, you should have a better idea of which plan is right for you.
Have fun with the rest of your planning!