Go to the Creation Hall. Hold L and press XYBAA. This will unlock Astronaut Template!, Knight Template!, Ninja Girl Template!, Spartan Template!, Super Girl Template!
Hidden music tracks
Get the hidden Pirate Coin in every stage. There's three hidden tracks unlockable at the bottom of the list in Isaac's item shop, marked with "?????". These are the Opening Credits (complete with animation), Mayor Rosa's song "Light of my Life", and the Ending credits song "Real Life", respectively.