I really liked the idea of having a Wedding Guest Book Tree, where guests placed their signature underneath their fingerprints. However, I didn't like the idea of asking everyone to put ink on their hands. I just found this on Etsy which will give us a guest book that we can hang on the wall after the wedding but without the messy ink:
http://www.etsy.com/listing/57957069/wedding-guest-book-tree-for-up-to-150