Ever since I released Qubes[1] I wanted to come up with a way of making some money without a lot of ADs or to charge the user to play the game. So this is what I came up with: If you beat the level with 3 stars I don't show you a AD, how about that?!
It's kind of the opposite of pay to win. If you win (with the optimal solution) you don't have to "pay".
Anyway, I hope this catches on and more developers adopt this strategy, and I also hope more people play my game.
[1] - https://play.google.com/store/apps/details?id=com.victorribeiro.qubes