Make your guest book a big deal. Give guests cards to fill out and drop in a suitcase as they exit the reception. You could also make tags that guests can sign and hang from a tree, or set up a booth with props to make a video guest book.
Had someone from Etsy make me a mahogany surf board, applied "just married" in vinyl for a wedding photo prop. It turned out totally adorable! When we have our reception, I plan on turning the surf board into the "guest book" so people can sign it.