Hocus is a minimal perspective illusion puzzle game by Gamebra.in, the game is based on M.C. Escher drawings and impossible shapes. The game is similar with Monument Valley, and it is an excellent way to sharpen spacial relations.
The game is not super hard, but it is just challenging enough to force your brain to slow down and relax. You will feel quite satisfied when that cube sinks into the hole. If you get stuck in the game, you could check out the video walkthrough for the game here from level 1 to 100.