We knew we didn't want a guestbook. The chances of us ever opening up this book again was next to nothing. We wanted to do something different (and truth be told we also wanted a picture of us on the wall without seeming too narcissistic!). So, we ordered a canvas of one of our engagement photos and I mixed in a little bit of my illustrations from our invitations. It's all printed on a canvas. We made sure to include a large black border. Then we bought a silver gel pen for everyone to sign our picture. It's a nice memento from our wedding that we will eventually hang on our wall.
If you don't want to use your own photo like us, use a framed art print and have your guests sign the matte. You can always change out the art when your style changes.
We bought ours through Miller's, but we had to go through a photographer friend. You can buy canvases at many places, just google "photo canvas" and you'll find tons.