A cup of Disney coffee. Yes, I know that is probably the least exciting "treat", but I absolutely love Disney's coffee. Extra points if it's a cold night while walking through the parks.
The Fish and Chips at Columbia Harbor House... it's pretty much become a tradition to eat there when in WDW. Honorable mention to churros and Okay I know that's three things, not one...
When I travel to Disney, if only visiting for one day, the one food I would have to have is the peanut butter cup @ the Confectionary on Main Street (Ice cream sandwich from plaza ice cream shop a very very very close second).
Right now I'd have to say the roasted chicken with red peppers and potatoes at Spice Road Table with a side of Pita bread to soak up the oil Then wash it down with a Grey Goose slushee or two in France