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.
This is how I plan on asking the maid of honor & bridesmaids. Really cute idea. 5 Creative ways to ask: Will you be my bridesmaid? This photo is a DIY balloon in a box that you have to pop to find a secret message inside!