Much of the city is accessible with curb cuts for wheelchair users. All the major sites (like the Met museum, the Guggenheim, the 9/11 Memorial and Museum, and the Lincoln Center) are also accessible. Some, but not all, Broadway venues have provisions for theater-goers with disabilities, from listening devices to wheelchair seating; consult http://theatreaccess.nyc.
Unfortunately, only about 100 of New York's 468 subway stations are fully wheelchair accessible. In general, the bigger stations have access, such as 14th St-Union Sq, 34th St-Penn Station, 42nd St-Port Authority Terminal, 59th St-Columbus Circle, and 66th St-Lincoln Center. For a complete list of accessible subway stations, visit http://web.mta.info/accessibility/stations.htm. Also visit www.nycgo.com/accessibility.
On the plus side, all of NYC's MTA buses are wheelchair accessible, and are often a better option than negotiating cramped subway stations. Taxis suitable to travelers with mobility aids are available through Accessible Dispatch (646-599-9999; http://accessibledispatch.org); there's also an app that allows you to request the nearest available service.
Another excellent resource is the Big Apple Greeter program, which has more than 50 volunteers on staff with various disabilities who are happy to show off their corner of the city.
Restrooms can be found in most department stores and the NYC parks website (www.nycgovparks.org/facilities/bathrooms) is a good source of info regarding bathrooms – some of them wheelchair-accessible – across the city's green spaces.
The city also provides paratransit buses for getting around town for the same price as a subway fare, though this service – called Access-a-Ride (https://access.nyc.gov/programs/access-a-ride) – isn't very practical for tourists as you'll need to attend an assessment appointment and fill in mailed paperwork before eligibility for the service can be confirmed (which can take up to 21 days). Visit the website for more info.
Download Lonely Planet's free Accessible Travel guides from http://lptravel.to/AccessibleTravel.