Founded in 1891 and now a National Historic Landmark, the New York Botanical Garden is the largest botanical garden in any city in the United States. It's also a well-known plant research and conservation society.
The Bronx Zoo, as you may not be surprised to learn, is in the Bronx. It's the largest metropolitan zoo in the US and one of the biggest in the world. Highlights include the Congo Gorilla Forest and the African Plains, and the Children's Zoo is a perennial hit with the little ones.