Not sure if all of these qualify as snacks on the dining plan.
Here are the gluten/dairy free snacks at MK and EPCOT that we have had or plan to have on our upcoming trip:
In both MK & EPCOT:
Cinnamon Glazed almond & pecan cart
Popcorn cart
Edy's frozen strawberry bar
Fresh fruit cups in various locations
Frozen Coke Slush
Frozen Lemonade Slush
MK:
Liberty Square Market- Cracker Jack, potato chips, craisins, fruit, veggies (corn on the cob, potatoes, sweet potatoes), lundberg rice chips, surf sweets, enjoy life and OMG cookies
Main Street Ice Cream Parlor- Rice dream ice cream, root beer float with Rice dream
EPCOT
Japan pavilion: Kakigori
Hope that helps!