This is the last of the four relief carvings on City Hall. The carvings "represent ... four principle concerns of city government: playgrounds, Schools, Parks, and Water Supply".
This one is the Playground one. Like the other three, this one has great detail. Faintly carved in the background are the flexibility rings, and either a pommel horse or the horizontal bars (on which I am a total klutz).
History Note: Today is Chicago's Birthday. It was incorporated as a city on March 4, 1837.