Brooklyn Bridge Park is huge and you'll find hundreds of free activities and events here each and every year, however Pier 6 is a great spot for kids thanks to its many attractions. Slide Mountain, several jungle gyms, Swing Valley featuring Tarzan ropes, a Sandbox Village, sand volleyball courts and even a full Water Lab are all available for free.
SOCRATES SCULPTURE PARK (always free) was once an abandoned landfill; in the 1980s, it was turned into an open studio and exhibition space for artists, as well as a neighborhood park. It is now an internationally renowned outdoor museum and park offering a host of free family programs and a place to picnic or walk along the East River with a view of Manhattan’s skyline.
WAVE HILL is a public garden and cultural center in the Bronx overlooking the Hudson River, with sweeping views west to the New Jersey Palisades. Admission is free from 9am to noon on Saturdays year-round and all day on Tuesdays from November through April.