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.
Shell Beach Destination Wedding Escort Place Cards Calligraphy on Mussel Shells
My kindergarten class made these "Messages in a bottle" for our "Under the Sea" Open House. They brought in one glass bottle from home and then wrote a message to their parents. Make wishes come true at graduation '