A great way to tell a Mom that she is a Grandma-to-be is to show up at her house on mother's day. For example, if you found out you're pregnant before Mother's Day, show up with a cake saying the name of all the mother's in the house... and then yours.
Recently, I read where a film star and his wife, each received a blue baby bottle for a gift. This was telling them they were going to soon be Grandparents of a baby boy. I thought this was a very touching gesture!