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.
LOVE this game. perfect amount of time needed to play without getting lost in it. Dev updates and improves regularly. adds features and content all the time.