Allow your guests to get creative and have fun with a photo booth. Guests take pictures of themselves with, or without, props and some photo booths even allow guests to write messages to the happy couple. Ask guests to include a copy of their pictures and a note in a photo book you leave on the side of the photo booth. For inspiration check out this real wedding over on Martha Stewart Weddings.
Leave an ink pad and a tree on a large piece of paper and have your guests fill in the leaves with their finger prints. Have them sign on or under their print to have a personalized guest book you can frame and display in your home. You can find pre-made thumb print trees on Etsy if you don't want to create one yourself.
Turn your engagement photos into a photo book by using one of the many photo processing sites such as Shutterfly, Costco.com, Mpix, etc. Or find a local photo development company to do it for you. Leave room on each page for guests to leave their sentiments. Or get creative and have guests give specific advice about certain things on every few pages. And if you are receiving professional engagements photos from a photographer make sure to ask them if they have any great sources for photo books. Odds are they already work with a company for weddings!
We first saw this book over on Emmaline Bride and had to go over and check it out for ourselves. Elegantly Bound has a custom envelope guest book where each guest takes a notecard out of an attached envelope and writes their greeting. They then put it back in the envelope and are done! At the end of your wedding day you can go and look in each envelope to see what everyone has said! How creative is that?
Whatever you decide, your guest book should be an item that you are able to treasure forever!