Artificial Intelligence
Artificial Intelligence, or AI, is symbolic of human ingenuity and progress. Less than 100 years prior to the start of the novel, many humans lived without running water and electricity. However, technology quickly enabled humans to have a virtual "helper," or AI, that allowed them make their lives easier.
Water
Water, which all of the characters in the novel interact with in one way or another, is a powerful symbol of the unknown. Humans are easily able to see the top of water, but not anything that lies under it. This creates an insatiable curiosity in many humans, who use that curiosity as a reason to explore what lies beneath the water.
Chicago
The City of Chicago itself is symbolic of the way many humans live in anonymity (especially in large cities). In the novel, the City of Chicago -- its large buildings and hordes of people-- allows the characters to hide themselves. This allows them to not only live an isolated life, but allows them to do largely what they please under the cover of large crowds.
Climate
Playground is an allegory warning against humanity's destruction of nature (especially the ocean). The novel shows how destructive human actions can be on the world, including things like pollution and intrusive exploration into areas that should not encroached on. The novel shows the delicate balance of nature, and warns readers that they should respect the world around them.
