Marty Shapiro wrote:

Also see the South Street Seaport Museum, with its aircraft carrier
and a Concorde. Web site is http://www.southstseaport.org/

I think you're confusing two different museums. South Street Seaport is on
the east side, near Wall Street. They've got a square rigger (The
Peking?), Ambrose Light Ship, and some other stuff. Worth going to, if
you're into that sort of thing. The carrier and the Concorde is the
Intrepid Air And Space Museum. As of just a few days ago, it's closed for
a planned 18 month renovation.

But, to answer the original question, there is unfortunately, no easy way
to get into Manhattan from any of the local GA airports.

From HPN, a 5-10 minute cab ride will get you to the downtown White Plains
train station. From there, it's about a half hour to Grand Central on
Metro North. Check http://as0.mta.info/mnr/schedules/sched_form.cfm for
schedules.

From TEB, I think you can get a NJ Transit train to Hoboken, then transfer
to the PATH train into downtown.

From FRG, somebody can probably run you over to the Long Island Railroad
station, then you can get a train into Penn Station.

None of these alternatives are what I would call convenient.