"One weekend a year, nearly 200 private buildings and structures around the five boroughs open their doors to the public.  Most offer free guided tours highlighting each site's history or notable features.  The roster includes a variety of monuments, historic buildings, parks, and even private homes (it also includes several places listed in this chapster, such as High Bridge Tower, the Highline, and Governors Island), many of which are seldom or never open at any other time of the year.  Our suggestion: browse the schedule online around the time of the festival, then try to check out as many as you can.  This is held early in October every year.  For more information, call 212-991-6470, or visit www.ohny.org." The Best Things to Do in New York.