A coin within a coin. A coinception. Almost all of the functionality of a D4, but flatter.
After printing, use a bit of oil to lubricate the connections. You'll want the inner piece to be able to spin several times if you flick it directly. This might require a bit of sanding.
To spin it, be sure to flip it in the air with your thumb striking the side of the coin that's away from the connection points. You'll be able to hear it if it spins correctly.