A few passages from Aquinas on the subject of play. First, from a question on whether play can be virtuous:
Just as man needs bodily rest for the body's refreshment, because he cannot always be at work, since his power is finite and equal to a certain fixed amount of labor, so too is it with his soul, whose power is also finite and equal to a fixed amount of work. Consequently when he goes beyond his measure in a certain work, he is oppressed and becomes weary, and all the more since when the soul works, the body is at work likewise, in so far as the intellective soul employs forces that operate through bodily organs. Now sensible goods are connatural to man, and therefore, when the soul arises above sensibles, through being intent on the operations of reason, there results in consequence a certain weariness of soul, whether the operations with which it is occupied be those of the practical or of the speculative reason. Yet this weariness is greater if the soul be occupied with the work of contemplation, since thereby it is raised higher above sensible things; although perhaps certain outward works of the practical reason entail a greater bodily labor. On either case, however, one man is more soul-wearied than another, according as he is more intensely occupied with works of reason. Now just as weariness of the body is dispelled by resting the body, so weariness of the soul must needs be remedied by resting the soul: and the soul's rest is pleasure, as stated above (I-II, 25, 2; I-II, 31, 1, ad 2). Consequently, the remedy for weariness of soul must needs consist in the application of some pleasure, by slackening the tension of the reason's study.
It is noteworthy, by the way, that this argument implies that the more contemplative and intellectual our work, the more important it is to play. He goes on to conclude:
Now such like words or deeds wherein nothing further is sought than the soul's delight, are called playful or humorous. Hence it is necessary at times to make use of them, in order to give rest, as it were, to the soul. This is in agreement with the statement of the Philosopher (Ethic. iv, 8) that "in the intercourse of this life there is a kind of rest that is associated with games": and consequently it is sometimes necessary to make use of such things.
He then notes three moral issues involved in play: (1) play should not be indecent or injurious; (2) while play should relax the tightness of the mind's harmony, it should not throw it out of balance altogether - i.e., while play is there to give us a bit of a break from perfect virtue and rationality, this does not make it an excuse to give up virtue and rationality altogether; even in taking it easy we shouldn't be taking a holiday from virtue and reason; (3) play should be appropriate to our circumstances. All three of these things require the direction of reason, however, so there is a sort of virtue that has to do with play as such; this he calls (borrowing from Aristotle) eutrapelia, which falls under the virtue of propriety (modestia); it is the virtue of "having a happy turn of mind, whereby one gives one's word and deeds a cheerful turn." It could also be called affability, pleasantness, etc.
If there is a virtue of play, there are concomitant vices pertaining to an excess of play, in which we do not have proper regard for the right circumstances or way of going about play, so that play becomes unruly; and to a deficiency of play, the vice of boorishness, in which a person becomes burdensome to others by being unable to appreciate moderate play. Boorishness is a lesser vice than frivolity, because the dangers of vicious play are more serious than those of being deficient in play. Austerity, insofar as it is a virtue, is in this sense closely related to eutrapelia, since such austerity opposes frivolity but does not exclude all pleasantness in life. Effeminacy and delicacy, on the other hand, insofar as they are vices, encourage cravings for excessive play and a general inability to do one's proper work when it gets difficult. In discussing the excess of play, Aquinas defends stage-acting, or more generally what we would call 'entertainment', "the object of which is to cheer the human heart," so long as it involves proper regard for circumstances and nothing indecent or injurious. (People who reward entertainers for indecent, injurious, or improper play, however, are sinning by encouraging vice. Decent entertainers who avoid the injurious and indecent, and have a good understanding of how to moderate their play according to audience, time, place, etc., are to be rewarded by us as a matter of justice, because their services are valuable.)