Well this is what I came up with. Mod Podge Mail.
They're made from cut up pieces of cardboard from one of the many boxes that came into my house, plus scrapbook paper, stamps that I uploaded from a website called alltheweb.com, mod podge and address labels. The address labels I actually made and printed on cardstock. They have funny names like Brent Bacon, Grandpa Grumps, Cousin Cookie ... you get the idea.
They took no time to make and turned out really cute. I may also make a batch and use plywood or something. We'll see how long these last in the destructive hands of a 3-year old.