I have never been on a big cruise, but we took our first river cruise last Nov./Dec. As far as clothes are concerned, bring warm casual clothing. One dress/casual outfit (nice top and pants) for the captain's dinner. No gowns, etc. Bring a fleece, turtlenecks sweaters, warm outter clothes.
We bought Christmas decorations. They have wood carving scenes that you will see often. I bought one - quite pretty. All the markets sell different mugs of gluwine or hot chocolate. I wish I had bought more mugs to put out this Christmas. As far as what to see, the tour will keep you quite busy. Go to the markets at night. They are very beautiful at night.
Enjoy your trip.