Principle: Play without a purpose for new or better experiences.
As you develop your game, engage in unstructured play to get a feel for the game's mechanics and the gameplay opportunities they offer. Approach it as a curious child would, by looking, observing, grabbing, manipulating, throwing, arranging, assembling, taking apart, and putting back together everything around you to develop an intuitive sense of how it all works.
While playing, allow yourself to think of new mechanics or ways to improve the existing ones. Is there some mechanic that stands out as particularly fun or interesting? Could a seemingly unimportant mechanic be brought to the foreground? Is there something about the objects in the game that could make for a new and interesting mechanic?
This is different from playtesting in that it's intuitive, unstructured, and completely informal. You're not looking for bugs, flaws or balance issues; You're just playing around while looking out for interesting experiences within and beyond your game in its current form.
Example: Will Wright's SimCity evolved out of a war game called Raid on Bungeling Bay. That game included virtual cities with factory buildings the player could bomb and destroy. In playing with the game's editor, Wright discovered that building new cities was more fun than the game itself. He continued to develop the editor, adding new features and making the world more dynamic, ultimately leading to the development and publication of SimCity.