Bottle Flip 2k16 is pretty mindless but fun.
There are tons of sudoku apps in the store. Mine is made by FingerArts and it lets you play offline.
Or download some podcasts to listen to.
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No microtransactions. One time payment to be able to save progress.
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Have spent hundreds of hours playing this over the years.
This may seem silly to some but a high fidelity Smash Hit. For those who don't know, it's a simple endless tunnel game where you throw ball bearings at glass targets and smash glass obstacles in front of you. It is absolutely killer on my GearVR, it's really the only app I use on it. There's something about the sound of glass shattering, the music, the visuals, it's just a brilliant experience that I keep coming back to. To be able to have desktop-level graphics, positional audio, and maybe a tweaked procedural course generator for endless mode would be amazing.