White painted stones by Natasha Newton - These would make a nifty place card. Just hand your guests a business card with a matching design on it. Then they get to match the card to the rock at their place. They could then take the rock home as a keepsake.
Such a simple thing, but a little hand painted stone (painted by you of course!) could be such a gift from the heart and costs next to nothing! (nothing if you already have a bit of acrylic paint and (Diy Garden Rocks)