I saw several people doing this last year and thought it was a really cool idea! Do you not have children, or want something just for you: do an advent with one of your snailmailers. Buy/make/draw/craft little presents and send it to each other in 1 big box. Agree upon a budget (if applicable), so one doesn't get way less than the other. You can do the whole month of december, or for example 10 days before christmas.
On Babble you can find some inspiration for packaging.