Fort Hancock at the north end of Sandy Hook has all kinds of abandoned gun batteries and turrets. You can get to a lot of them, but some are fenced because of years of decay. Also, there's the old Eastern State Penitentiary in Philadelphia...it may be closed during the winter though. The website is
www.easternstate.org. BTW, Eastern State has a great haunted prison tour during the Halloween season called Terror Behind the Walls that's definitely worth checking out.