The specialty at the Parkside Wagon is churros. For the uninitiated, a churro is a Mexican pastry consisting of a long stick of fried dough rolled in cinnamon sugar. They are very tasty. You can also purchase cold drinks, although either The Gazebo or the Royal Street Veranda, both close by, are better choices if all you seek is liquid refreshment.
Churros are, as you might imagine, difficult to find in Japan, and when the Wagon first opened the lines of people waiting to buy churros rivaled the lines to get in to Space Mountain. Consequently they started offering the treats in more locations, and that, plus the novelty probably wearing off, means the lines have shortened over the years.