I'm thinking of re-painting my other Christmas market stand to sell cotton candy... I can take actual cotton balls and dye them pale pink and blue, make little clear bags, and have bags hanging up for sale, plus of course the stuff on sticks. Or like you said the hot dog cart might be good, create a clear plastic box, with a big bowl... I'll need to look for another hot dog cart, though!
Btw, I went to TRU and bought the new ice cream cart, plus some useful stickers from Michael's. So updates coming soon!