This optical illusion card idea comes from Yellow Bear in NZ. It uses a simple A-2 sized card, but with the center of one side cut out. The trees are the things that keep it together. All of the stamps are from the White Pines Cardmaking technique workshop that I taught (don't miss out next year!).
Inside, I stamped another Christmas tree in the center. You can stamp another sentiment inside, as well as sign the card. It adds more color to the card using the multiple colored stamping (dark outline plus inside color).