An advent calendar doesn’t always have to be a lot of filled boxes, why not simply mailing a card a day?
It is more exciting for the recipient if the cards put together form a poster.
Photojojo shows how to make a poster with 24 cards from a photo. I think that you can also use a self-designed Christmas card for an advent calendar.
And on the back you can write a nice, amusing or inspiring aphorism (apart from the address ;-)).
Links:
Bei Photojojo: How to Turn a Photo into a Mailable Mosaic
Google search result for ‘aphorism’
Google search result for ‘aphorismus’
Here at unikatissima: Boxes not only for the advent calendar