Here's an adorable Santa advent calendar. This is simple to do with young children and doesn't require much prep work at all.
You fill in Santa's beard one day at a time with cotton balls. By Christmas Eve, Santa will have a full beard!
All you need is the printable Santa page from here, cotton balls (I cut mine in half to fit better), and some glue.
Here's Bekah putting on her first cotton ball. Happy December!