Postcrossing mail, specifically. Something other than bills, advertising circulars,and catalogs.
Send a postcard to someone in the world, and soon you will get a postcard from another stranger in return.
Such as a beautiful one from Japan:
Mount Fuji and cherry blossoms! A classic!
Or how about one from Russia:
You just can't beat it.
I will try to be more on time and post early tomorrow!