Guest book idea - great because you can get a photo of everyone at the wedding and get some advice for a scrapbook. Such a funny idea. It'd be fun to combine this with a photo booth. A photo booth guest book.
Years from now, you could flip through an album of well-wishes and scan all the handwritten names. Or you could flip the switch on your computer and watch your guests in action. Devote a corner of the event space to a video photo booth: Set up a digital camcorder, leave a movie clapper and dry-erase pens for guests to write their names (or tidings), and let them raise a glass and talk a blue streak. For a seamless background, hang a large white piece of fabric or paper.
I love this idea. An envelope guest book. You just take a store-bought scrapbook/album and glue envelopes into it. Lay out colored pieces of paper for guests to jot notes down on, and then they can place them in the envelopes.