Steel mailboxes make a great outdoor mosaic project and can be mosaiced with a variety of materials, including found objects. This pique assiette mailbox was mosaiced by my neighbor with pieces cut from patterned china plates:
Relevant objects such as brass house numbers can be incorporated directly into the mosaic itself, and you might choose to do the entire mosaic in found objects such as souvenirs made of durable materials (avoid wood and plastic). On the other hand, ordinary vitreous glass mosaic tile can be used in the regular way to render figures such as flowers or people or trees or entire scenes. Or you might decide to write the street number or family name in mosaic tile. The point is that a standard-size mailbox provides enough space on the side to accommodate a variety of designs.
The mailbox itself should be unpainted steel, preferably galvanized, with no plastic clear coats that will interfere with bonding.
At Mosaic Art Supply, we use thinset mortar, a type of sticky concrete, for our outdoor mosaic projects, and I have written some instructions for using thinset for detailed mosaic artwork. If you would rather not have to mix up thinset, then you could use an outdoor construction adhesive such as Liquid Nails Outdoor Formula or an epoxy, but these materials have fumes and might not be as durable as thinset. We use thinset for mounting tiles in outdoor mosaics, and we also use thinset for grouting the project as well.