This is by far one of the best advent calendars I've seen. It is 24 envelopes clothes pinned to twine. The twine is hot glued to two wooden disks. The smaller disk is held to the wall with Command strips. I love love it.
Every day we open an envelope to reveal an activity for the day. We've done things like make sugar cookies, visit Santa, make snowflakes and other awesome activities. I can get you our full list if anyone is interested.
We got the idea from Design*Sponge. You can use this PDF to print off numbers of your own. I printed mine on sticker paper, which made it easy to cut them out and adhere them to the envelopes.
I'm considering painting the clothes pins for next year. I think aqua would be cool but may just stick with white.