I was thinking of a reward system, where the user will have 1 key at the begining which can be used once to reveal a solution of one level and then will disappear. A new key will be obtained if the user solves a level with the exact or less number of moves the solution requires. So one can obtain 90 keys for all levels + 1 key that is given initially for total of 91 keys max. Of course that means that I need to also keep score and introduce some competitive goals...
Btw, thanks XDAdevelopers for the review of my game!!!