I just took a quick peek at the link provided by Jean. A couple of good choices I noticed right away are "Deck the Halls" (my favorite non-religious Yuletide carol), "Carol of the Bells," and Mel Torme's "Chestnuts Roasting On an Open Fire" (properly entitled "The Christmas Song," if I'm not mistaken). Another obvious choice would be the venerable "Jingle Bells." Then there's "Winter Wonderland," "Let It Snow," "Silver Bells"...

Hopefully, a thorough perusal will prod your memory and help you come up with a sufficient number of familiar "classics" that are appropriate. I would tend to stick with well-known, well-loved singalong stuff rather than made-up PC neo-pagan ditties like "Goddess Rest You Merry Gentlepersons" or whatever.