I love this idea where the children put a cotton ball on santa's face. I did this a few years ago for the class and also my little girl. I got a large sheet of paper and painted santa's face. We did it as a whole class.
Here is a link for a template
Below is my feeble effort of painting santa's face on a large sheet of paper. At least this way you could if necessary leave out the numbers so a child glues a piece of cotton wool on each day leading up to Christmas. Good if you have more than 24 in your class.
One teacher I worked with did a fabulous one where she got a bunch of twigs and basically stuck them to the wall with a big ribbon around the bottom. She then got the children to draw a picture and put their picture in a numbered envelope to hang on the branches
The paper chain advent calendar isn't a new idea but I thought I would mention it just incase someone doesn't know about it. Here are some instructions and a poem. You could write an activity on it each link for the children to do each day.
If anyone has any photographs or ideas then please email me and I would be happy to put them in this link