Some of my favorite, less often performed, traditional carols not mentioned above:

Allons, gay bergeres (French)
Beata viscera (12th c. Latin, by Perotin[us] and Philip the Chancellor)
Gabriel from Evene King (13th c. English)
Jesus Christ, the Apple Tree (Ingalls' setting)
Nesem Vám Noviny (Czech)
Over the Hill and Over the Dale (words: J.M. Neale)
Shepherds, Rejoice (William Billings' "Boston" setting)
Tau garço, la durundena (Catalan)
While Shepherds Watched (Daniel Read's "Sherburne" setting)

"Jolly Shepherd" (by Ravenscroft) is a nice round.

To counter the cloying sentiment of the season you can sing "The Bitter Withy".