What about an apothecary jar full of signed stones?
Start by adding some moss at the bottom of a large clear vase or jar. Select smooth (and preferably flat) stones light in color. Keep a Sharpie nearby and voila! Whenever a guest graces your home, have them write a message on a rock and place in the jar. Or at the conclusion of a baby shower or wedding reception, presenting this jar to the honoree is a beautiful reminder of those who were in attendance.
Pretty cool, right?
Sidenote: As much as I love this idea, I still have little kids running around my house and I fear that any kind of an open container with rocks inside invites some bad decisions :). Save this idea for a more adult home or if you can trust your kids.