Each day you open the Amazon appstore there's a little parcel icon if you click on it you get a few Amazon coins. I've had 3 each day for the last few days. I'd not opened the appstore for a while before then as the choice is limited but it's better than nothing! I've only tried on Android, not sure if it's the same on iPhone.
You can only use the coins on apps and in app purchases.
Thanks to HerefordCiderApples for pointing out that you might have to manually install it from Amazon as it's not on the play store.