Paper ephemera, yes! Alas, I have a mailart budget.
I think, having had the job of clearing out a house that had been continuously inhabited by three generations of the same family, I would like to not leave a lot of old paper around so look for genuine antique ephemera in my mail art, starting with the box of "I don't know who these people are" photos dating from the 1880's to the 1960's. Somedays I wish I had salvaged more paper, more days that I had salvaged less.
But, everyone, if you do leave photos of people — write the names on the back!