Coney Island has long been New York City’s popular beach and playground destination every summer. It has been called “New York’s/America’s/People’s Playground” since at least te early 1900s. More specifically, it was called “America’s Mad Playground.”
     
“Mad playground” was printed in the Buffalo (NY) Enquirer on September 4, 1909. “AMERICA’S MAD PLAYGROUND, CONEY ISLAND” was printed in the book Loafing Down Long Island (1921) by Charles Hanson Towne.
 
   
Wikipedia: Coney Island
Coney Island is a residential and commercial neighborhood and entertainment area, located on a peninsula in the southwestern part of the borough of Brooklyn in New York City. The neighborhood is bounded by Manhattan Beach to its east, Lower New York Bay to the south and west, and Gravesend to the north, and includes the subsections of Sea Gate to its west and Brighton Beach to its east. Coney Island was formerly the westernmost of the Outer Barrier islands on the southern shore of Long Island, but in the early 20th century it became a peninsula, connected to the rest of Long Island by land fill.
 
The origin of Coney Island’s name is disputed, but the area was originally part of the colonial town of Gravesend. By the mid-19th century it had become a seaside resort, and by the late 19th century, amusement parks had also been built at the location. The attractions reached a historical peak during the first half of the 20th century. However, they declined in popularity after World War II, and following years of neglect, several structures were torn down. Various redevelopment projects were proposed for Coney Island in the 1970s through the 2000s, though most of these were not carried out. The area was revitalized with the opening of MCU Park in 2001 and several amusement rides in the 2010s.
     
Newspapers.com
4 September 1909, Buffalo (NY) Enquirer, “Star,” pg. 7, col. 1:
The first act passes with the candy shop, and the second at Coney Island on a summer’s night with all the resort’s gaieties in full swing, the scene showing the water chutes, the helter skelter, the sideshows, the jolting camels, merry-go-rounds and other minutiae of the mad playground.
 
Google Books
Loafing Down Long Island
By Charles Hanson Towne
New York, NY: The Century Co.
1921
Pg. 129:
AMERICA’S MAD PLAYGROUND, CONEY ISLAND
 
Newspapers.com
8 May 1924, The Crenshiaw County News (Luverne, AL), “Things Unusual” by T. T. Maxey, pg. 3, col. 5:
Coney Island has been referred to as “America’s Mad Playground,” and the stranger who visits it on a hot summer night readily and naturally gains the impression that the swarming hordes of people are mad for play, also that almost everybody is going to or coming from Coney Island.