Blue play dough
Directions: Mold blue play dough into the shape of luchot (Torah tablets). With a toothpick, have your kids carve each name of your Shavuot guests. Luchot tablets are thought by Jewish scholars to be rectangular. According to tradition (Shmot 24:10) the luchot were formed from a sapphire/lapis blue stone. http://en.wikipedia.org/wiki/Lapis_lazuli
Let play dough air dry overnight and you will have beautiful place cards/tablets and a momento for your family and friends to keep.
This can also be done with FIMO, and baked after letters are carved.