My favorite guest book idea captures the faces of all the people who attend your wedding. Rent an old Polaroid camera and have your guests take a photo as they arrive at the venue. They can write a little message on the bottom of the photo and add it to a book for you!
A new wedding trend is the "finger print/family tree" guest book. Allow guests to build a beautiful work of art by placing their finger prints on a family tree.
Use your engagement photos as a guest book. Create large images of your engagement photos and put them on some card stock, let guests to write their 'well wishes' for you and then hang the photos in your home later.
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A questionnaire book is another personalized guest book idea. Head over to Shutterfly and make a photo book, inside the book add questions, "How many kids will the couple have?", "When will they buy their first home?" "Leave advice for the bride and groom." etc. Allow your guests to have some fun and answer some questions about you while looking at photos of you.
So, have fun with your guest book! Don't just do the old fashioned "sign your name here" guest book. This is just another way to make every, single part of your day special and different.