My mom is very good at purchasing greeting cards for her grandchildren and sending them on the holidays. Not only do they love receiving mail but they especially love opening the cards and discovering the cash tucked inside.
I appreciate the fact that mom sends cards and that my kids reap the benefit of her generosity and thoughfulness.
I too enjoy sending cards to friends to encourage them. I'm sure that not only do they appreciate them, but they wish that there was some cash stuffed inside. That would require them becomming one of my mother's grandchildren, and that's just not going to happen.