An analogy can be made between mental fitness and physical fitness, and while I may run into trouble later trying to push this analogy too far, let's see just what can be accomplished before it gets silly or stupid.

Mental "Diet" and "Exercise"

Just as in physical fitness, mental fitness, leaving out the questions of psychiatric illness and psychological well-being, involves a sort of mental "diet" and "exercise", such that the things a person concerns themselves with involves both a process ("exercise") and a set of content and context ("diet").

Immanuel Goldstein's Gym (Est. 1948) Where Our Motto is "2+2=4!"