Why didn't I think of using an old water bottle to store colored pencils in? Lots of cute ideas of things you can mail that are under 13 ounces (meaning you can stick stamps on and throw in the mailbox!
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.