It's not just a cliché: New York really is the city that never sleeps. Although you could spend a lifetime here and still not see everything, even with 24 hours to spare you can still pack a lot in, according to Lonely Planet staffer and NY native, Ali Lemer.
Put on your walking shoes and start out early on the Upper West Side with a take-out breakfast from Zabar's, an old-world delicatessen that's a NYC institution. Grab a bagel with cream cheese and lox and head east to the intriguing, esoteric American Museum of Natural History, which is housed in a massive 19th-century Beaux Arts building off Central Park West. Join the ever-present herds of local schoolchildren gazing up in awe at the gigantic blue whale (it's a model) suspended from the ceiling of the Ocean Life hall, and take a quick gander at the dinosaur fossils upstairs before you leave.
Those with an astronomical bent can head next door to space out at the Rose Center for Earth & Space (a combined-visit ticket is available); otherwise keep your feet firmly on this planet with a stroll through the urban oasis of Central Park. Head south from the museum to Strawberry Fields, the flower-strewn mosaic memorial to John Lennon that's found in the park opposite his former residence, the Dakota; there may even be an impromptu Beatles jam session for you to join in.
Continue south down the paths of Central Park, past the sunbather-filled 15 acres of Sheep Meadow, to Columbus Circle, where you can do a bit of window-shopping at the Time Warner Center, which has upscale shops of every stripe as well an enormous Whole Foods Market on the lower level. Grab an inexpensive lunch at Whole Food's cafe (and then treat yourself to a truffle at Godiva Chocolatier on the ground floor). The center is also home to Jazz at Lincoln Center, which offers top-notch jazz against a glass-wall backdrop with amazing views over midtown; while you're there, stop in and buy some tickets for the late show that night.
Walking south on Broadway will take you through the Theater District to the neon jungle of Times Square, down to Herald Square, home of world-famous department store Macy's, and on to Madison Square Park, where New York's iconic triangular 1902 skyscraper, the Flatiron Building, stands guard just to the south. A few blocks further down Broadway, you can duck into ABC Carpet & Home to browse their six floors of decorative housewares, and then continue on to Union Square, which hosts a year-round greenmarket every Monday, Wednesday, Friday and Saturday, with tent stalls bursting with fresh fruit & veg, baked goods, cheese, meats and flowers.
Grab a snack from the greenmarket and have a browse in some of the shops around the square: Filene's Basement for discount designer clothes; Forbidden Planet for sci-fi toys and comics; Footlight Records for musical-theatre vinyl; and the legendary Strand Books, where you can find almost any book under the sun in its labyrinth of shelves.
By now it's late afternoon, so take the 4, 5 or 6 subway from Union Square down to the Brooklyn Bridge/City Hall stop; forget about a visit with Hizzoner (New Yorkese for the mayor) and instead take a walk across the Brooklyn Bridge, one of the great wonders of New York since its completion in 1883. If you just can't get enough of those unforgettable views of the Manhattan skyline twinkling against the dusk, you can keep an eye on it over dinner at the River Cafe (), located right on the water underneath the great span. Or else try one of the other (less expensive) local eateries, such as Grimaldi's for pizza or Bubby's for burgers and fried chicken.
After dinner, take the A or C train at High St back to the 59th St/Columbus Circle stop for that jazz show, or else take a nighttime stroll through the stately townhouses of Brooklyn Heights to end up at the Brooklyn Promenade, a long stretch of riverside pavement that will allow you even more time with that mesmerising Manhattan view.