Mickey ice cream bar or ice cream sandwich
Dole Whip
Mickey shaped waffle with strawberries and whipped cream
Anything from Goofy's Candy Co or the Main Street Confectionary
Brownie Paw from Kusifari Bakery at AK
ANY cupcake at Starring Rolls (I like the marble one the best!!)
Of course the dole whip from Aloha Isle
candy apples from Sunset Blvd. Candy Store in HS
and there is a wonderful drink in the France pavillion at Epcot that I cannot remember the name
Lets not forget the Christmas specific snacks. You did mention you were going to be there Christmas Week. Gingerbread from American Adventure in Epcot (Wonderful) springs to mind.
Dole Whip (Aloha Isle in Adventureland)
Funnel Cake (American Pavilion in Epcot)
Mickey's Premium Ice Cream (Pretty much anywhere)
Anything from the Bakery in France (Epcot)!
Number 1 must do for me every trip is a Mickey Ice Cream Bar. But there is a lot of other good things. I've heard only good things about Dole Whips in Adventureland and i will be sure to try one on my upcoming trip. Just about anything from Main Street will do you right too.