Great song! Yes, I agree that A minor has a depressing sound, but combined with a nice major chord it is as if the heavens have just opened
Personally, I find that "O Come, O Come Emmanuel" sounds beautiful on acoustic guitar.
Also, the "12 Days of Christmas" is a fun and upbeat one to play even though it is repetitive (I usually play around with the strumming to keep it interesting).
^^These are both quite easy
All of Relient K's Christmas songs are pretty awesome as well! <-- (Though some are a little difficult in terms of tuning and such)
Have fun playing guitar!