We have sailed three Merrytime cruises now, though none over Christmas itself.
On our cruises they had gingerbread house making to sign up for. We did and had a lot of fun. They use a kit for the house itself and have plenty of decorations. You get to keep the house but are not allowed to take it off the ship -- so you get to keep it in your stateroom for the duration. It adds a pleasant smell ! And is a yummy snack if the vultures in your room decide it is worthy.
SW