Idle Slayer is really cool. Love the art and the progression is fun! Also one of the few "idle" games that you can actually play. It's a simple platformer, but you earn while the game is off and upgrade and change the play style.

Shattered Pixel Dungeon is a cool rogue-lite game with great mobile controls. Even if you don't have much time, it saves your run's progress so you can do a few floors here and there.