Brooklyn Botanic Garden is a 52-acre garden beside the Brooklyn Museum and Prospect Park. Designed by the Olmsted Brothers and opened in 1911, it includes a Japanese garden, the Cranford Rose Garden, a children's garden, and one of the best cherry blossom displays outside Japan.
It is a strong stop in spring, but the formal gardens and glasshouses make it useful in most seasons. Pair it with the Brooklyn Museum for an easy half-day around Prospect Heights.