Two parts here; a day trip further afield and potential wandering in New York City.
A daytrip to Philadelphia
The best trip I know to make from New York for some fun heritage rolling stock, about as "heritage" as you'll find in day-to-day service, is the cheap way to Philadelphia - which is one of the most interesting destinations you can reach, too. It costs $21 (£13-14) each way, from a clerk or vending machine in the New Jersey Transit concourse at Penn Station and at any Philadelphia station.
You take New Jersey Transit to Trenton, NJ, which could be any of a great variety of stock, often in combination; some very new double deckers, single deck carriages between 5 and 30 years old, and 1960s MUs.
At Trenton, you have a cross-platform connection onto the SEPTA (South East Pennsylvania Transport Authority) train to Center City Philadelphia. SEPTA run exclusively "silverliners". These are single-end self-powered electric cars made from stainless steel that normally run in 2s or 3s. They mostly date from 1963 or 1974-6, with a few from 1967 too. They all retain their original interiors, and reflect the fashions at their times of construction for things like fake wood panels and faux-leather bench seats.
At the stations between Trenton and Philadelphia, you will observe the practice of serving low platforms by means of the conductor, while the train is in the process of decelerating, opening the door and lifting a "trapdoor" under which their are steps to allow boarding from ground level. You can also have a good front view if you're unobtrusive enough about standing behind the cab.
Here is a picture from the slightly less exciting 1970s type (hopefully in at least one direction your train will have at least one 1960s car, which are more fun), and a picture from low platform operation of a different type of train:
At Philadelphia, there are a whole lot of things to do; I recommend a decent guidebook, or failing that, getting of at Suburban or Market East stations and seeing what takes your fancy.
For rail interest, there is the four-track downtown tunnel with "trolleys" on the outer tracks and the Market-Frankford el on the middle express tracks; the former become street-running trams to the west of downtown, while the latter becomes an elevated line with views of the city to the East and West.
More interesting than these in my opinion, though, is the PATCO speedline, which makes a very scenic bridge crossing into Camden, NJ, with heritage stock with an even more kitsch 1960s interior than the Silverliners. Even reading it up on wikipedia is entertaining:
http://en.wikipedia.org/wiki/PATCO_Speedline.
For variety, you could spend a bit more on the faster Amtrak Northeast Regional in one direction - perhaps going back in the evening, as slow trip on the local trains is less fun in the dark. In that case, you'll travel on a late 70s/early 80s streamlined amfleet carriage, most likely behind an AEM-7 electric locomotive from about the same time.
Some subway exploration
I couldn't recommend a day trip to Philly highly enough. Another worthwhile day out, though, with just an $8.50 metrocard, is a tour of the elevated subway lines in Brooklyn.
Good elevated lines include the West End Line (the N train on the Coney Island end), the Brighton Line (the Q train and B train on the Coney Island/Brighton Beach end) and the Brooklyn Broadway and Myrtle Avenue lines (The J, M and Z trains). An interesting sight is the abandoned Myrtle Avenue el, which is now an empty frame running above the Broadway el at Myrtle Avenue Station, where the M branches from the JZ. I took this photograph of it:
Spend some time in Coney Island, which not only has a throughly impressive 8-track (I think) subway terminal, but is also a nice beach, which is busy with a impressively diverse crowd of people and has fairground rides if you're into that.
A longer but very rewarding trip is the Brooklyn Broadway el to Broadway Junction, and thence the A train to Far Rockaway. Far Rockaway is good for a wander down to the beach, then afterwards take the LIRR Far Rockaway Branch back to Jamaica, Atlantic Terminal in Downtown Brooklyn or Penn Station for an additional fare.
A fun curiosity on the subway is the single-track Franklin Avenue Shuttle, which runs with 2-car trains partly on elevated track and partly in an open cutting, and has valiantly resisted many attempts to close it. It serves the Brooklyn Botanic Garden, which is a vey worthwhile attraction.
Heritage on the New York Subway
The best heritage stock on the Subway is the 1964 R32 Stock
(wikipedia), which have their original bench seats and 3-part rollerblinds. You'll find them on most runs on the C train, which runs local on the 8th Avenue Line. If you don't get a set of R32s on the first C train that arrives, the next probably will be one. For more heritage stock that's now been retired from service, I highly recommend the Transit Museum in downtown Brooklyn, which is marked on the Subway map.
'll get my coat.
