I do not know when the concept of Advent Calendar started but it is a good concept for toys. In the market as far as I know only two toyline used this concept especially during Christmas season. One is Playmobil while another is Lego.
What it does is simple, you are given a paper backdrop as diorama, they are 24 compartments to allow kid to open each compartment each day to find some new surprises. That would keep the children busy.
The main playmobil is no other but Santa and the rest are the animals and other accessories in the forest. How's that for the idea for fun to kids? Apparently they are really popular and Mario from A year of toys got a set of it and doing a review daily from it till Christmas.
If you like smaller parts (not recommend for smaller children due to small parts), you can pick Lego advent Calendar. For that "Why did I Buy that" is doing the countdown till Christmas day.
I must say it is an ideal gift to children. I know I would love to have one even though I am no longer a child.
You can get Playmobil Christmas Advent Calendar or Lego advent Calendar for Christmas. :)
On the side note, I have just updated Christmas banner to get into the mood of the holiday season. :D