Part of a much larger mural along Rogers Avenue. If you look at the mural in Street View, you can see that Google's automatic face-blurring software has blurred the faces of almost everyone honored in the mural!
It's another FDNY communications dispatch office (here's a better comparison picture — look how similar they are!), or at least it was until 2008. Fittingly, the Russell D. Ramsey Memorial Triangle is catty-corner to this structure.
I had never heard this before, and I can't find any confirmation of the claim, but I hope it's true! There's not much left of Ebbets Field — its former site is occupied by high-rise apartments.
Something else I learned from this informative wall: "Before its original 39 acres were designated for a botanic garden in 1897, the Garden's land was covered in trash, construction rubble, and ashes from its history as a city dump."
As indicated, this innovative playground is located between Lexington and Third Avenues, between East 122nd and East 123rd Streets, in Manhattan. This sign, however, is located about nine miles away, near the intersection of Washington Avenue and Eastern Parkway, in Brooklyn, at the entrance to Dr. Ronald E. McNair Park.