I know the whole point is to keep it a secret.
I'm having too much fun with it so I thought I'd tell you what I've been doing.
Most mornings, I stop at McDonalds (hush, I know) for a Yogurt Parfait.
I go through the drive-thru and when I get to the window to pay, I also pay the order for the car behind me. I ask the cashier just to tell them I said Merry Christmas. As I drive away I can see the cashier trying to explain to that person and the dumb founded look ... then they look ahead to my car with smiles and waves.
Something so inexpensive and easy. I'm getting a kick out of this and I hope it helps these people start the day off with a little smile, and a feeling of some holiday cheer.
Give it a try. I bet you'll feel just as good about it as the person in the car behind you.