See a defunct station
The old City Hall stop in lower Manhattan was envisioned as the MTA's most majestic station when it was completed in 1904. It eventually closed because longer chains of subway cars couldn't fit onto its looping track. The New York Transit Museum offers tours to its members, although it's possible to get a gratis peek at the beautiful platform. Simply stay on a downtown 6 train as it makes the loop around Brooklyn Bridge--City Hall to the uptown track, and keep an eye out for the bygone station's vaulted ceilings and Art Nouveau skylights.
Peep underground art
Thanks to the MTA's Arts for Transit program, the subway system is filled with colorful installations intended to beautify the often dreary stations. Though acclaimed artists like Roy Lichtenstein and Sol LeWitt have contributed to the program, some of its best curios were created by less well-known artists: For example, in the Newkirk Plaza station on the Q line, gaze up to spot David Wilson's blue-and-red patterned Transit Skylight installed in the ceiling. Or check out our favorite piece, Bill Brand's Masstransiscope, a zoetrope created from 228 hand-painted panels. Take a ride on a Manhattan-bound B or Q train from DeKalb Avenue to view the kinetic mural between that station and the Manhattan Bridge. (Be sure to look to the right.)
Look for abandoned tracks
A quiet stretch of Joralemon Street in Brooklyn Heights conceals one of the transit system's hidden gems: A brownstone on the block (near Willow Place) is actually a facade housing an entrance to the defunct Atlantic Avenue Tunnel, the city's oldest railroad passageway. (The DOT discontinued tours of the space in 2010.) Farther east, there are four disused tracks and two unfinished platforms hidden in the Utica Avenue station in Bedford-Stuyvesant. The depot was part of a planned expansion of the IND subway line, but the MTA abandoned the project because of a lack of funds. You can still see the station shell by standing on the existing platforms and looking up—you'll be able to spot the underside of the tracks.