In lieu of the traditional guest book, I love the idea of a wish tree. I love the details of this particular tree with the seashells in the vase and the lovely orchids on the branches. The photo via Project Wedding. Although I love orchids, I would probably use more affordable flowers such as daisies, plumerias, and small flowers and leaves just found here and there.
The Thoughtful Tree from Martha Stewart Weddings is more playful than elegant, but I like the idea of using using doves for wishing the couple peace and happiness.