A bit older, but still great IMHO: Sanctum 2 and its DLC go on sale for $3 every sale. I’ve sunk over 450 hours into this FPS TD hybrid. The DLC is worth getting as well as it doubles the content of the game.

Titanfall 2 is about $5 and offers a diverse and creative SP campaign with mixed soldier and mech combat. I’ve played it at least 5 times.

Mirror’s Edge is $7 with a robust parkour mechanic and alright, if not a little dated story. I’ve also played this at least 5 times.

For more casual offerings: A Short Hike, Alba, Firewatch, Smushi Come Home, Röki, Journey, Turnip Boy, Scanner Sombre, Deepest Sword, Yono, Snakey Pass, Baba is You

Other Games I’ve enjoyed: Portal 1&2, Full Metal Furies, Superhot, Borderlands 1&2, Mothergunship, Echo, High On Life, Crab Champions, Gunfire Reborn, Strange Planet, Hades, Hellblade Senua’s Sacrifice