Talos Principle is currently 80% off, along with other CroTeam games like Serious Sam.
This is a very challenging puzzle game, sort of in the vein of Portal. You're trying to navigate mazes with the help of various devices: lasers, electronic jammers, totally-not-companion-cubes, fans, etc, all while receiving messages from an omnipotent god-like voice. Probably around 100 puzzles, plus crazy-hard custom ones you can download if you find the game too easy.
If you think you have amazing spatial reasoning skills I'd recommend this game to take you down a peg.