I've been using them as backgrounds for collage elements. All of my collage bits of paper and ephemeral are also vintage and of the some era.
It's been a lot of fun, mostly tongue in check humor, and a touch of irony. After I alter the postcards I seal them with a protective acrylic finish, low gloss to preserve the vintage look. You can find these altered postcards in my Blue Valentine Etsy Shop. Oh, and there's lots more to come.