A small, deep frame for displaying small objects - with or without a picture taped onto the back. The hole in the frame measures 50mmx50mmx18mm.
There's a picture hanger on the back that works well with a standard bulletin board pin. You can also hang it with Loctite Fun Tak.
(The mini backhoe in the picture is this remix: https://www.thingiverse.com/thing:2813563 )