When I visited my mother yesterday she wanted to stop in at Hobby Lobby, and we spotted these adorable plain brown houses just right for Daisy and her Little Animals.
They are sturdier than cardboard (they're labelled paper mache), and the roofs come off like lids, and you can color them.
And they're very inexpensive--I think the largest was $7 (and there's always that coupon code . . .) The timing was perfect for Daisy, as she had recently acquired a goat postman. Now he's got somewhere to deliver the mail