Sorry, musicman!

You did say "upbeat" though, which is the only fault I can find with the suggestion of In the Bleak Midwinter.
How about some American songs, which are so much less familiar in NZ? I never heard Silver Bells until I came to Japan (you could cut out or rewrite the one mention of snow)?
There are some nice Shaker songs which are simple but moving, but my book is buried somewhere so I can't give you a specific recommendation.
Musicals - did you mean Christmas-themed ones, or any musicals?
(exits humming "I'm Your Dentist"...)