I saw this cute idea in a magazine years ago. I cut it out and hung it on my inspiration line and didn't do anythign about it until this year when it was screaming at me to take action. :) It was easy, fun, and I can post it on my bog now because it won't ruin the surprises!
You just take 25 envelopes decorate them any way you want, hide fun surprises inside, and either hang it on your wall like a tree or mail it to your friends and suggest that they hang it like a tree. :) I have one tree like assemblage and then some up close photos of the envelopes. You still have time to make one of your own!