There are so many things that you should try. I'll list them by parks
MK
-Dole Whip Floats from Aloha Isle. It's pineapple and vanilla swirled ice cream in pineapple juice. Really refreshing on a hot day
-Mickey Premium Bar from an Ice Cream Cart. They are at every park, but its more magical eating it in MK
-Cake Cup from Chesire Cafe. It's basically 90% frosting but it is amazing
EPCOT
-Strawberry Shortcake from Sunshine Seasons.
-School Bread from Norway Bakery. This is the EPCOT "It Dessert" Its a donut like pastry with coconut and vanilla and it is the best thing you will ever have.
-Kaki-Gori from Japan. It's shaved ice but better
If you are going during either the Food and Wine festival or the Flower and Garden Festival there are more snacks that are all equally good.
HS
-Red Velvet Cheesecake Cupcake from Starring Rolls. Enough is said about it. But just to let you know Starring Rolls does close early
-Carrot Cake Cookie from Writer's Stop. It's like a carrot cake whoopee pie
AK
-BBQ from Flame Tree BBQ. It's real BBQ that is all I have to say
There are other things you should try, but this is the best of the best.