Sam Wang and Sandra Aamodt have an article in Dana Cerebrum (here), Play, Stress, and the Learning Brain.

They define play.

First, play resembles a serious behavior, such as hunting or escaping, but is done by a young animal or is exaggerated, awkward, or otherwise altered. Second, play has no immediate survival purpose. It appears to be done for its own sake and is voluntary and pleasurable. Third, play occurs when an animal is not under stress and does not have something more pressing to do.

If that is play then it occurs among many animals: mammals, birds, some other vertebrates and even some invertebrates (octopus, squid, honeybee). This wide spread points to a very long history and suggests it serves a vital purpose. Another indication of usefulness is that play is fun; enjoyment is a sign of survival traits. Play is rewarded with dopamine. It is tailored to the lifestyle of the animal. Depending on the typical behavior of the animal there are three types of play. Playing with objects is typical of species that hunt, scavenge or eat a variety of foods. Locomotor play is usual in active animals who run, swim, fly, climb trees. Social play (fighting, chasing, wrestling) uses pretending in animals who have important social encounters. It is the species with the bigger brains for their body size who most engage in social play.

So play has an adaptive purpose or purposes. What would they be?

play is practice that prepares animals for the real activity later—when it matters. … In mammals, play is necessary for forming normal social connections. Rats and cats raised in social isolation become incompetent in dealing with others of their kind and typically react with aggression. In our species, abnormal play as children often presages dysfunction in adults. A notable feature of psychopaths is that their childhoods lacked in play. … Play also transmits culture. … Risk taking in children’s play may be an important developmental process. It tests boundaries and establishes what is safe and what is dangerous. … play also helps children learn what they like and don’t like.

Interesting, playing is a low stress activity (either play lowers stress or stress interferes with play). This is important for learning during play.

Play activates other brain signaling systems as well, including the neurotransmitter

norepinephrine. … Norepinephrine is also involved in rousing us to attention and action, but by acting as a neurotransmitter. Norepinephrine facilitates learning mechanisms at synapses as well. In some neurons, norepinephrine improves brain plasticity, such that change becomes possible when this chemical is present in elevated amounts. The same is true for dopamine, which accounts for how reward leads to long-term changes to make us want more—neural plasticity mechanisms are strongly facilitated when reward occurs. … Though real-life stressors trigger the release of both epinephrine and cortisol, play does not increase cortisol. Cortisol is a stress hormone that helps us in genuinely dangerous situations by redirecting resources to the most urgent needs, such as repairing a wound or fighting an infection.

For humans, play continues into adulthood.

work in adult life is often most effective when it resembles play. Indeed, total immersion in an activity often indicates that the activity is intensely enjoyable; this is the concept of flow, or what athletes call being in the zone. Flow occurs during active experiences that require concentration but are also highly practiced, where the goals and boundaries are clear but leave room for creativity. This describes many adult hobbies, from skiing to music, as well as careers like surgery and computer programming. Such immersion can make solving a great challenge as easy as child’s play.

And not just humans play as adults. Dogs do and they have a special signal to commence play so that their actions are not misunderstood. It is a similar signal in many animals and young of different species have been seen playing together.

What has this to do with consciousness? It seems to me that play is like a type of consciousness – a way of experiencing the world that can be turned on and off. We are able to experience a low tension, enjoyable, somewhat pretend world in which we actively experiment, discover and learn.