Nonocross - a nonogram puzzles app with some default boards and a very simple design.
Pekka Kana 2 - an old wacky 2D platform with you commanding a rooster going from start to finish of every level.
SuperTuxKart - a classic by now, a complete parody of the original Crash Team Racing with characters being mascots from various projects and important elements of the Linux ecosystem.
(not on F-Droid) OpenRCT2 - a re implementation of the Roller Coaster Tycoon 2 engine. Free and open source, but requires the game’s files.