Kosho Uchiyama wrote about "sesshin without toys" and how zazen is not playing with anything.
"It seems to me that we spend all our lives playing with toys. ... Doing zazen means to actualize the reality of life. zazen is the self which is only the self of the universe, without any playing with toys. Zazen is like the time just before our death when all the toys have been taken away. Yet, even then we look around for something to play with, if only for an instant."
(Opening the Hand of Thought, p. 62)
"Among all the human activities in the world, there is nothing in which we can live out our own life without amusing ourselves with toys. Only sitting zazen is free from self-amusement with toys. This is the point where zazen is wondrous."
(The Wholehearted Way, p. 127)
What I find missing is to say that zazen itself is a toy, a device, that they keep playing with, to the exclusion of other tools (recitation, repentance, etc.). While reducing Zen to zazen seems minimalist, there is one more step. See what Bankei says:
"If the Buddha-mind is clearly realized, that's enough. You need do nothing else — no practice, no precepts, no zazen or koan study. Nothing like that. You'll be free from care, everything will be taken care of, just by being as you are."
(The Unborn, p. 116, tr. Waddell)
"The only way any of you can become unborn and realize the Buddha-mind is to confirm what I'm telling you in your own mind. I won't tell you that you have to practice such and such, that you have to uphold certain rules or precepts or read certain sutras or other Zen writings, or that you have to do zazen. I'm not going to try to give you the Buddha-mind either — you already have it. If you listen carefully to me, and grasp the Buddha-mind that's already yours, then you become a genuine living Buddha. Wherever you are standing, that place is the Unborn. Whatever you want to do, you can do it. If you want to recite sutras or do zazen, observe precepts, recite the Nembutsu or the Daimoku, you should do it.59 If you're a farmer or a tradesman and you want to work your farm or your business, then go ahead, do it; whatever it is, that will be your personal samadhi. My part in this is simply to tell you about it and to try to get you to confirm the Buddha-minds you were all given when you were born."
(The Unborn, p. 120, tr. Waddell)
"To exert yourselves in religious practice, trying to produce enlightenment by doing religious practices and zazen, is all wrong too. There's no difference between the mind of all the buddhas and the Buddha Mind of each one of you. But by wanting to realize enlightenment, you create a duality between the one who realizes enlightenment and what it is that's being realized. When you cherish even the smallest desire to realize enlightenment, right away you leave behind the realm of the Unborn and go against the Buddha Mind. This Buddha Mind you have from your parents innately is one alone—not two, not three!"
(Bankei Zen, p. 76, tr. Haskel)
"Now, you may be doing zazen and reading the sutras, but abide in the Buddha Mind that you have from your parents innately, just as it is, and realize the Unborn. If you practice zazen or read the sutras with some deliberate aim in mind, hoping to accumulate merit, or whatever, you'll only be changing the Buddha Mind for merit, or changing it for zazen and sutras! That's how it is, so all you've got to do is acknowledge with profound faith and realization that, without your producing a single thought or resorting to any cleverness or shrewdness, everything is individually recognized and distinguished of itself. And all because the marvelously illuminating Buddha Mind is unborn and smoothly manages each and every thing."
(Bankei Zen, p. 85, tr. Haskel)
"All of you should realize the vital, functioning, living Buddha Mind! For several hundred years now, [people in] both China and Japan have misunderstood the Zen teaching, trying to attain enlightenment by doing zazen or trying to find 'the one who sees and hears,' all of which is a great mistake. Zazen is just another name for original mind, and means to sit in tranquility with a tranquil mind. When you do sitting meditation, you're simply sitting, just as you are; when you do walking meditation, you're walking, just as you are."
(Bankei Zen, p. 96, tr. Haskel)
"Mind accords with all circumstances, yet doesn't arise or cease
The sages of old praised this, calling it zazen
Blind people wear out their cushions waiting for enlightenment
Just like trying to make a mirror by polishing a brick"
(Bankei Zen, p. 123, tr. Haskel)
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)