What Midtown actually is
Midtown (roughly 34th to 59th Street) is the commercial core of Manhattan — office towers, hotels, theaters, flagship retail, and the tourist infrastructure built around them. It's the most visited part of the city and, for residents, often the most avoided: expensive, crowded at all hours, and short on the neighborhood character that makes other parts of the city interesting to live in.
Times Square
See it once, at night, briefly. The light and scale are genuinely impressive the first time. Eat and drink elsewhere — the restaurants immediately around Times Square are priced for foot traffic, not quality. The TKTS discount theater ticket booth in the center of the square is a legitimate reason to be here during the day.
Grand Central Terminal
One of the great public interiors in the world: the main concourse's vaulted ceiling, the constellations painted in turquoise and gold, and the quality of light through the arched windows are worth stopping for even if you're not catching a train. The lower-level food market is good for lunch. Free to enter, open most hours.
The New York Public Library
The main branch on 42nd Street — the one with the famous marble lions out front — has one of the finest Beaux-Arts interiors in New York, including the Rose Main Reading Room, a 78-foot-high vaulted space with long oak tables that is still an active library reading room. Free to enter; bring ID to access materials.
Fifth Avenue
The retail flagship corridor between 42nd and 59th Streets: Saks, Bergdorf Goodman, global luxury brands, Apple, and a lot of foot traffic. Worth a walk for the streetscape and building facades if not the shopping; the stretch near Rockefeller Center is particularly architecturally coherent.
Navigating it
The Midtown grid is the most legible part of Manhattan — numbered streets running east-west, avenues north-south, addresses reliably indicating block position. Most of what visitors want to see is walkable from the major hotels, but the walking takes longer than it looks: Midtown blocks are long, sidewalks are crowded, and the distance from Times Square to the MoMA, for instance, is a solid 20-minute walk in good conditions.