My idea is similar to the white board idea. I saw someone else do it. You can use mini chalk boards and have people write messages with chalk. I am a teacher so I love the idea of chalkboards!
Other ideas:
A branch in a vase where people hang wishes for the bride and groom
A photo book guest book- where you use polaroid cameras or a photo booth that makes double prints (one for you and one for the guests) to take pictures of all of the guests and then they write a comment
The picture with a mat around that people sign
A glass object that means something to you that people say or a ceramic object.
I love the postcard idea too, so you can get mail when you get home.
Definately something to consider. I was also wondering about flying a photographer in or just going with the hotel photographer. Thanks for the advice ladies!
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Good luck in your search!