I agree with the PP - Dole whip Float (not just a dole whip, which is just the ice cream) - Available at Aloha Isle in Adventureland
Figaro Fries for a more salty/snacky treat - Available at Pinocchio's Village Haus in Fantasyland
Homemade Mickey Mouse Rice Crispy treats with the ears dipped in chocolate - Available at Main St Bakery on Main St.
Also, just a quick stop at the Main St Confectionery is a good bet for finding a delicious treat!