Available on PC, Steam (if you’re willing to pay), and android (through F-droid, not sure about play store or any of the others):

Shattered Pixel Dungeon. Run though randomly generated dungeon floors. Every 5th floor, fight a boss. I suck at it, so I’ve only gotten past the 2nd boss once, but that’s because I suck. Currently 5 different classes with a 6th in development (unless I’m wrong and it’s already out), each playing differently for the most part. You’ve got a warrior, rogue, huntress, mage, and duelist, with all but warrior being locked until you do easy enough requirements to unlock them for subsequent runs.

Can pause, exit, and come back to a run whenever during a run.

Available on PC and android (both F-droid and play store);

SuperTuxKart. Cart racer with a lot of fan made content like maps and carts. Has varying difficulties, but the lower the difficulty the lower the max speed in a race, if that matters to anyone else besides me. Not exactly full-on single-player since there is the option of online play, either against friends or random players if you find an open lobby.

Can pause pretty much any time offline but backing out makes you have to restart a race from the beginning. Same thing applies to the multi-race cup things.