There is a surprising number of gems in F-Droid. Simon Taltham's Puzzles is excellent, as is Breakout 71. Shattered Pixel Dungeon is fun too, and of course I live emulating games with Retroarch. Lichess is neat and has a lovely Android client if you play chess.
I haven't finished it, but I thought Xeonjia: Ice Adventures was pretty good. It's a combination of sokoban and adventure games where you navigate the world by solving block puzzles.