The millenary Game of GO, with a hint of Artificial Intelligence.
This is an app to play the Japanese game of Go. The game allows to play against the device (cellphone, tablet, etc.) using some algorithms from Artificial Intelligence. Another option is to play against another person in the same device.
The Game of GO is so simple that a 5 years old child can learn to play it, but it requires one lifetime to master. Practice or get better playing GO, and develop your own abilities of abstract thinking, memory, attention, problem resolutions, space orientation, etc.