I’m a fan of Freedroid Classic, a FOSS remake of a commodore 64 game called “Paradroid”. You’re a robot on a deserted space ship full of other malfunctioning robots, and you have to hack / shoot all of them. You start out as the worst robot, but if you encounter a better one, you can hack it to take over its body – if it doesn’t kill you first.
Takes a few runs to get the hang of it, but it’s a lot of fun.