Small icosahedron that you can print as a hollow shell and slip something inside. It's sturdy enough that it wouldn't break easily, but not so strong that you couldn't break it if you wanted to get what was inside - so good for enclosing a small note or gift, maybe?
See the instructions for slicer settings.
UPDATE: new file uploaded, for better print quality on top and bottom layer.
I used a custom MakerWare profile starting from "Low PLA", with the following settings changed:
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