I like this idea for a garden or an indoor decor. You could paint just about anything on the rocks (faces, fish, etc.) and let the kids help. Or even names - say a gift for Grandma with a grandkids' name on each rock.
"Stones and Rocks DIY Wedding guest book". Guests write messages to bride and groom on rocks with Sharpie instead of signing traditional guest book. This would be cool to put in giant glass vases on either side of the fireplace once the wedding was over.