As the title say how about a decoration with Greeting cards. Everyone in the each send and receive greeting cards on various occasions. Instead of piling up the cards in one corner think about taking out for this Christmas.
I would suggest decorating the XMas tree with greeting cards. Many of the cards will be in a open type. So, we can have them pasted with two side tapes. If you have enough cards you can also made them pasted on the door / wall to make them more attractive.
Hope you will try this idea on this Christmas. I promise that I will come up with more Christmas ideas.