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.
Gurgle (wordle clone) and Gridle, word games. Cards With Cats is very good if you like to play Hearts. Ball2box, it's all in the name. Super Retro Mega Wars has some serviceable classics like asteroids and Tetris. SuperTuxKart is alright. Simple Solitaire Collection isn't maintained anymore, but hasn't been surpassed yet on f droid imo. Getting good mileage out of Lemuroid of course, the console emulator for people who don't like configuring things.