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.
Picture-Perfect Birthday Party - Photo Wall -- Print photos in various sizes. Lay them on the floor in the shape of the birthday number to determine your arrangement. Then tape them to a wall.