I'm going on a RCI cruise with my Mom and sister for Christmas, this will be my first cruise, as well it will be their first cruise. We have read a lot about what to expect on a cruise, but have not heard about how the experience might vary with the holidays. Are the menus different? Do the staff expect more in the way of tipping?
One of the nicest suggestions I've ever heard as a "gift" for staff is pre-paid phone cards. I think that came from CruiseAholic.
There will probably be a traditional "turkey & all the trimmings" menu offering on Christmas day. Otherwise, it will probably be pretty much the same.
It would be fun to decorate your door or string lights on your balcony if you have one. (Don't forget the extension cord and some duct tape.) You could decorate your door as a giant gift, complete with bow. A roll of wrapping paper, some tape, and ribbon wouldn't take up too much space. You could also tape a holiday banner to your door.
You might want to bring a box of Christmas/holiday cards to use as "tip envelopes" and tip on Christmas Day rather than at the end of the cruise.
As a gift for Mom and Sis, let me suggest pashmina shawls. Great for chilly nights or too much A/C in the dining room. Check E-bay. They don't take up much room (important) and are beautiful.