I'm a big fan of Duolingo and have a vision of building something similar for any topic - not just languages.
The leagues are fun to ascend through and exp feels quite satisfying to gain. I've probably played a lot more than if it was just learning without all the sound-effects and funny animations. Which part makes you want to play/learn more?
Aside from Duolingo, are there any tricks you use to make you want to keep learning?