Sometimes the easiest exercise is also the hardest. When building your esoteric muscles you want to build long term focus and concentration.
The most intense exercise is a simple meditation. Sit in a comfortable position. Take three deep bellows breaths, close your eyes, and count down from 100 to 1 while visualizing each number.
Yes, as simple as that. If your thought strays, start over. Focused attention is the name of the game.