It’s hard to recommend something without knowing the makeup of your family and what they’re interested in but there are a few of the many possibilities.
Empire State Building: Observations Deck, Simulator Ride (I’m not sure if this is still around), also has a kosher pizza shop.
Ice Skating: Central Park, Bryant Park and Rockefeller Center
Statue of Liberty
Lots of Museums and Art Galleries (not all family friendly)
Lots of parks, central park alone can take up an entire day
Shopping, I wouldn’t recommend actually buying anything in Manhattan but there are lots of cool stores