Heard of a site called Duolingo? It’s a language learning site some of my friends use. It has some interesting gamification elements I think you should look into.
You get points and level up for playing lessons, and you get streaks for playing a lesson each day, for example.
As there get to be more lessons, and the lessons get harder on this site, you might find elements like that help to keep players going.
I also recommend the Extra Credits web series, for ideas on gamification and game design in general.