This app was mentioned in 4 comments, with an average of 1.00 upvote
You can create a end-to-end puzzle game solver. In order to make it hard, you can create a framework for solver and later on use that framework to solve 2-3 different puzzle games.
First try the brute force way. Since that would work only for simple puzzle instances, you would be forced to think about clever way to optimize your solution. You will have to use Artificial Intelligence like algorithms like A*, etc.
The puzzle games I think most suitable for such a framework are like below : https://play.google.com/store/apps/details?id=com.rafaelwmartins.pushbox&hl=en
https://play.google.com/store/apps/details?id=com.reactor.pushingmachine
Build a solver for grid based puzzle games. Bonus point : separate out the common function in a separate library which you can use with any other game.
E.g. games like Sokoban : https://play.google.com/store/apps/details?id=com.rafaelwmartins.pushbox&hl=en
I would suggest making a puzzle game solver algorithm. First try the brute force way. Since that would work only for simple puzzle instances, you would be forced to think about clever way to optimize your solution.
The puzzle game I thought of solving was this : https://play.google.com/store/apps/details?id=com.rafaelwmartins.pushbox&hl=en
https://play.google.com/store/apps/details?id=com.reactor.pushingmachine