The Dole Whips in MK (Aloha Isle) are $3.99 ($4.25 w/tax) and are very good and a refreshing snack. The lines are long though!
I took a walk one day in Pop's foodcourt and paid attention to the snack offerings. You can get a jar of Tostito's Cheese Con Queso, a large bag of Doritos, Tortilla chips, Potato Chips, banana, small boxes of cereal, celery w/peanut butter, celery/carrot sticks w/ranch, whole large boxes of Entenmann's donut holes or 8 large donuts, cookies, mickey mouse rice krispies treats w or w/o chocolate, some gluten free snacks also, plus your orange juice, cranberry, apple, milk, soda, water. There were pastries too.
There is a list in the dining plan area here on the boards that lists all of the snacks for every park. I printed it out and took it with me, but found I didn't even need it. Most things were marked or the CM's would tell you what was covered.