Not sure if you needed snack CREDITS or just "snacks" so some of my picks aren't on the dining plan but worth it.
MK- dole whip float, pepper jack cheese pretzel, loaded tots at Friar's nook (shared this between 3 of us), ANYTHING at the confectionery
Epcot- curry chicken pockets at China, nudel gratin at Germany, sweet pretzel (kringla) at the Norway bakery, caramel corn at Karamel Kuche in Germany, check out the bakery case at Sunshine Seasons. My fave is the turtle brownie bar. Buffalo popcorn over by Figment. And poutine (sometimes specialty flavors too) at the refreshment port in Canada.
Animal Kingdom- blueberry cheesecakes pod at Satuli Canteen, mickey cinnamon roll at Kussifiri Bakery, chocolate covered pineapple spear at Zuri's sweet shop, pulled pork cheese fries at Flame Tree BBQ, buffalo chicken chips (might be seasonal) at trilo bites in dino land, man and cheese at eight spoon cafe.
Hollywood Studios- carrot cake cookie (yes i've had the "new one" and it's still a favorite), huge ice cream funnel cakes over by the lake, sweet cream cheese pretzel (found this by the frozen show), totchos at woody's lunchbox, and the lunch box tart too!