Every year, our kids look forward to the Playmobile Advent Calendar. The calendar basically has two parts, both revolving around 24 little playmobile children, Santas, pets, and other Christmas-oriented toys: (1) a stand-up sheet to which parents attach 24 little boxes, one for each day of December until Christmas, and (2) a little tableau, where the kids can set up the toys as they open them. Each day, the kids get to open a box and place the toy on the tableau, setting up a scene that will be finished by Christmas Eve.
This page refers to the 2005 Calendar, which is an indoor scene of kids, pets, toys, a fireplace, Santa, etc., all gathered around a Christmas tree. December is half gone, but I'm sure if you get one, your kids can catch up.
I guess that I have two religeous caveats regarding these calendars. First, while opening a little present each day is a big treat for the kids, I suppose it doesn't have a lot to do with "the true meaning of Christmas." Second, these aren't technically true "advent calendars," since they cover December 1-24, rather than when Advent happens actually to fall for any given year (and religion), but I promise your kids won't mind.