Don't always run when they should, but we've managed to get around a little.
On the bank of the river Main opposite the Romerberg where the Christmas market is based there was a vast flea market. I was reminded of a similar market in Vienna, where it is said that all goods stolen during the week appear. Seeing someone trying to sell a teenage boy a racing bike that was far larger than he could ever happily ride made me wonder. Probably a good place for curio shopping.
In some ways Frankfurt reminded us of London with it's large river and mix of old and new, plus very strong multi-ethnicity.