Well, I have a lot of favorite snacks at Disney, so hope you don't mind my mentioning a few...
Disney popcorn is the BEST. I don't know why, but it is perfect. I like to get it before parades/fireworks at MK in a cart right at the hub, and I also never fail to have it during Fantasmic!, where they have people walking it around like at a ballgame.
Sleepy Hollow in Liberty Square (MK). We usually get a funnel cake (DD's tradition), but I haven't tried the relatively new waffle sandwiches they are offering now. There is a nutella and fruit one and a spicy chicken one, and I think a ham/cheese type one that sound out of this world. I have seen pics and they are HUGE, definitely meal-worthy.
Cappuccino cupcakes at Pizza Planet (HS). Not everyone does, but DD and I also really like their pizza.
Kaki Gori at the Japan pavilion at EPCOT. It's like the best snow cone you ever had - I love the melon flavor and DD likes a rainbow of flavors. She also loves a watermelon slushy drink in Morocco.
Turkey legs at MK at a cart in Frontierland. Not everyone enjoys sitting on a bench and hogging down a big hunk of meat, but they are very tasty and juicy. Better than the ones outside Dinoland in AK.
Speaking of meat, I haven't tried it yet, but the pork shank at Gaston's Tavern in MK is on the bucket list. We do love our hunks of meat!
Have a wonderful trip, and happy snacking!

Now I'm going to be thinking about this all day and I'll let you know if I think of anything else!