I made this one into a Christmas countdown calendar. I just painted the old cookie sheet red and glued on paper to create the design I wanted. I used mod podge over the whole thing to seal it up. The scalloped square is just a piece of chipboard I had laying around in my scrapbook supply box, I painted and decorated it and glued magnets (I used advertisement magnets that I cut up) to the back. This part moves from day to day. The last two days are glass magnets that I made (using cut outs from magazines), one with Santa and one with Jesus.
You don't have to just make advent calendars. Since they are magnetic you can create a message board, a picture keeper, a place to display your child's art for the day, a weekly meal schedule, etc. The options are only limited to your imagination.