Well potatoes and corn, of course!
As you can see, the same cooking method and supplies are used for the side items as for the crawfish...a rig that can boil water, a basket, and a large piece of wood to lift that basket out of the boiling water. Dump the good food into a cooler, throw some butter and Tony's on the top and shake it up.
I have heard that some folks boil the crawfish, potatoes, and corn all together in one pot and then dump it out together. Not sure why these guys did things separately, but I trust them.