Little packages of Candy kisses (Tootsie Rolls, or other such candies) always work! OR, you can make a scavenger-type game for each day for the kids to follow together, and then have a little bundle of kisses at the end for them. Or you can research the picture each of the 12 Days of Christmas and have the kids learn the lyrics of the song each day. At the end of the 12 days, you could give each of them a special little gift. Hope these ideas get you started!
Usually the countdown starts at the beginning of Dec. You can buy advent calendars with little candies already in them, or some just have pictures. I have one that was a train, with little doors to open to see the pictures. I have another that had little books. The favorite was a chimney type with little boxes that I put tiny toys, money, candies, etc in - different stuff in each box, or parts of sets - like little army men. Technically, the 12 days of Christmas are after Christmas, from Dec 25 to Jan 6.
My mother used to buy me little calenders that cost only about $2 - $4 & they used to tell little stories about Jesus & things like that. Now that I look back and I found it very cute! You can even buy them at regular everyday stores! Or you can purchase them online.
I got this idea someplace but; can't remember where. To make a paper garland using red and while paper....makes it Christmasy. Make it for as many days that you want to mark off. Each day...take off one of the garland circles. Then the kids can count what's left and know how many days till "Santa" will come. Hope this helps:)