I wish I and seen this thread last year when I decided to make one. I wanted a calendar that could actually hold a treat or small gift, so I came up with this:
#2 Box Advent Calendar by karjor - Cards and Paper Crafts at Splitcoaststampers
using the Stampin Up Sizzix #2 box die. It was a big hit, and I carefully packed it away - can't wait to get it out again and fill it up. Additional closeup in my gallery.