With search not functioning, I'm having trouble trying to figure out if this was a variant of a previously available encounter or not… Instead of getting my tree from checking the leaderboard this year, after helping out with building snowmen once I got the following noncombat:
One of the student participants in the Abominable Snowman contest flags you down across the quad. "Hey, thanks for helping out and Merry Christmas!"
He hands you a package, which proves to contain a rather nice artificial Christmas tree.
You found: fake christmas tree
Edit: And actually, I was also able to get a tree/chisel from the leaderboard as well, after helping out a bit more.