I have been begging every couple I work with to do this for their wedding and it's finally going to happen: a fingerprint guestbook. This was one of the first things I found when I started my Pinterest site and I absolutely love it.
Here's the basic concept: either find a friend or some super-talented artist on Etsy to design a large print of a tree with lots of branches. The branches are bare until your guests use colored inkpads (to match your wedding colors, of course!) to put their thumbprint on the tree, which then become the leaves on the tree. Then you can leave it as is, or they can sign their name on top of the thumbprint. There are tons of ways to make it individual; having your initials 'carved' on the tree, having your two thumbprints sitting in the middle of the tree as lovebirds, or even making it a car or Up-style house instead of a tree with balloons lifting the car/house off of the ground.
It's so neat to have a beautiful keepsake from your wedding that you can and want to display prominently in your home that has the imprint of everyone special to you in your new shared life.