Nintendo recently changed the rewards on MyNintendo, and today they added a new option for getting coins on a regular basis.

Users who opt into receiving Nintendo marketing emails will receive a 30 Platinum coin reward for opting in, and 10 coins each month so long as the opt-out is not taken. For those who have it disabled, you can re-enable it by clicking on your Mii in the upper right corner and going into Account Settings.

The monthly email joins visiting the eShop and Miiverse on the Wii U / 3DS, signing into My Nintendo and visiting Nintendo's website while logged in as sources of coins that do not utilize Miitomo.