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.
These are a nice decoration that can be made with recyclable paper to take away the guilt of takedown or just reuse them when you table needs some color! They are also great on a wall or dangling over a window.