How much do they already know about Christmas? Do any of them already celebrate it?
If they're interested in making conversation, they'll probably also be interested in seeing your pictures. If you happen to have pictures with you showing your family/friends participating in holiday traditions, then they can pose their own questions, try to describe what's going on, and go off on tangents.
Also, you can get into a discussion about the values behind this holiday and where they exist in the scheme of local holidays (for example, it's a time to travel and be with family, and they probably do that for some of their big holidays too).
Singing carols is always a good idea, because the verses are made to be easily remembered/catchy, they can go home and teach them to their families (if they want), and usually they are surrounded by traditions (wassailing, Christmas pageants) and contain them as well (christmas trees, roasting chesnuts, santa, shepherds, etc).
Also, depending on the nature of this class, you could sort of have them plan a party on a massive scale with a fictitiously large budget. Who should they invite? What kinds of food should they serve? What decorations will you need? How many days will it take to set up? What should the dress code be? And so on. Then maybe you could pare it down and put on a more intimate Christmas pot-luck or something when the day finally rolls around.
You can also discuss Santa Claus... his origins and so on. But if your students are advanced, then you can share this list with them: http://www.main.com/~anns/other/humo...csofsanta.html
When I was in school, that was a very popular memo that circulated through a few different classes and always got a laugh. Maybe they'd be interested in knowing the physics beind Santa? To find other versions of this, just google "Santa" and "sonic boom."
Please do update us on what you end up doing, especially what you think worked well-- I'm really interested in how this class turns out!