I do not mean to put Christmas on your mind before Thanksgiving, but this is something you need to create before December 1!
Last year inspired by a project I saw on Flickr, I made magnetic advent calendars for my nieces and nephews and even myself! Instead of using cookie sheets, because I did not have any from garage saling, I used sheet metal memo boards, which I had on hand from selling at craft shows. Because they were Christmas gifts, I am anxious to find out if they use their calendars this year!
With non-aluminum cookie sheets or jelly roll pans, you decorated the back so you hide the magnets behind. Then, each day you pull a magnet out and put it on the front; it is a sugar-free advent calendar! With mine being on flat sheet metal, the magnets are in a tin so the kids will have to pick through (they can practice their numbers, too!).
Comb through your junk drawer and missing pieces box to find things to use. I found pesos, bells, buttons, erasers, dice, dominoes, game pieces and bottle caps. Then, I used cardboard, ribbon and scrapbook items to fill in the rest. You can view my photo notes on Flickr of what and how I used the pieces!
To make it into a group swap project, get a group of five people. Each person would be assigned five days of the calendar (1-5, 6-10, 11-15, 16-20, 21-25) and would create five sets of five designs (25 in total). Each person could decorate her cookie sheet (the back/bottom) and then you swap out your magnets for the others. If you are afraid of a color disaster, agree to a certain colors ahead of time. For mine, I used red, pinks, greens, white, silver and gold.