DH would say a frozen banana at any park (so would my dad!) and then also a Dole Whip float. And probably Tonga Toast - since Kona bf is his 1 big must-do each trip.
I'm tougher! Popcorn or a Mickey ice cream bar from Main St. would remind me of being a kid. Mickey waffles are certainly an only in WDW thing . . . but I don't know that I have just one food. Maybe instead, as soon as the car or bus passes under the WDW welcome sign on the highway, I'm set!