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.
Set a shipshape table with seaworthy vessels whose sails designate guests' seats. These whimsical place cards require little more than kraft paper and a simple folding technique. Outfit boats with Life Savers candies and cast them adrift on blue napkins.