Plenty of cities have a harbour. Sydney is its harbour - that's the difference. Forget the idea of a downtown heart: all this town's vibrancy seems channelled straight towards the deep indent of water guarded by the Opera House on one side and the Sydney Harbour Bridge on one the other. Sightseeing made simple. You can do it all without straying more than 10 minutes from the tooting ferries and wheeling seagulls of Circular Quay. 11 p.m., Touchdown. Magic - the harbour view, all shimmering with lights, and the glass of cool Clare Valley Riesling that goes with it. Mentally it's only lunchtime. So, even though the last notes have been sung at an open-air performance of La Boheme and the crowd is trickling home from the green parkland of the Domain, bedtime seems far off. Time to stroll, then sit at a bar by the water's edge and drink in the view.
The Opera House defies postcard familiarity. What a daring, fresh, fantastic building! No matter which angle you see it from, it commands attention, with those roofs like billowing sails (though credit is also due to the person who said it looked like nuns in a rugby scrum). The Harbour Bridge seems modest, almost puny, by comparison. Beyond, ferries glowing like fireflies crawl across the black water.
8 a.m., What Summer? It's Saturday morning, at the height of summer, but the sky is grey - bleak. Horribly fit-looking people in shorts and skimpy T-shirts (yes, out at this ungodly hour) are putting up umbrellas.
9 a.m., Recce: How much can be managed in a couple of hours, without breaking into a canter? On a saunter around The Rocks, site of the original penal settlement, the first stopping point is the gallery of Ken Done, Sydney's most exuberant artist. He treats the Opera House to the sizzling colours of surfing gear; makes the famous roofline look like leaping dolphins.
Pause to pick up useful bumph in the old Sailors' Home that is now the Visitors' Centre (the Sailors' Thai Canteen, a fun place to eat, is also in this building). Then stroll on past the wharves of Circular Quay, past the swanky new apartment block nicknamed The Toaster, to the gate of the Royal Botanic Gardens. A Dublin girl is driving a hop-on, hop-off trolley that takes people through 74 acres of urban oasis on a scenic tour. Exotic palms, splashy flowers, stern ibis and fruit bats hanging like tattered handkerchiefs high up in the trees. No hopping, 20 minutes.
11.45 a.m., Fish Market: Out with the map and off to the massive, factory-like sheds of the Sydney Fish Market at Darling Harbour. Glossy, yellow-finned leather jackets, green-lip mussels, "choice headless octopus", Moreton Bay bugs like truncated crayfish and prawns the size of bananas - there they lie in obscene profusion. More awe than appetite.
1 p.m., MCA cafe: The tables outside the market, where fish-chomping shoppers fend off enormous, red-billed gulls, aren't for featherphobics like me. Instead, back to Circular Quay for lunch with an old pal at the MCA Cafe where Neil Perry, well-known chef of Rockpool, oversees the Museum of Contemporary Arts kitchen. Modern, cleverly eclectic Australian cooking - and all the suppliers of fresh produce are credited on the menu, in a Darina Allen sort of way. The clouds are breaking up, unclogging that hole in the ozone layer that has people plastering themselves with factor 35. The sun brings clowns, jugglers and human statues out on to the prom. Verna, who has been in the gym for half the morning, sets off to walk the three miles home. "It's like that here - everybody walks, everybody's fit," she says. "It's the open-air life - the fact bodies are so much more on show." And the Olympics are still to come.
3 p.m., Manly Jetcat: Why settle for a taxi and busy Bondi when you can skim across the water to Manly? Developed more than a century ago as a posh holiday spot for Sydneysiders, it's now part of the Northern Suburbs - but it still has unbelievably clear water and clean, fine sand, separated from the beachfront strip of surfing and rollerblade shops by a row of tall Norfolk Island pines. Half the pleasure of going there is the 20-minute trip out and back. The harbour unfolds like a huge blue flower, its frilly edges tipped with the pale gold of a dozen beaches. How many Eddie Irvines and Bonos have cliffside villas and gleaming yachts in this enormous playground of a city? Before you know it, you're back in town, Manly sand still crunching between the toes.
6 p.m. Bridge Walk: There's an organised climb of Sydney Harbour Bridge - a giddy, roped-together, group ascent of the outside of this high arch they call The Coathanger. No thanks. My wimpish alternative is to climb one set of stairs from Argyle Street, another from Cumberland Street, and cross the bridge on a pedestrian walkway that runs parallel to the eight-lane highway. Even peering through the safety netting is disconcerting: 16 construction workers lost their lives erecting this shortcut from south to north shore in the early 1930s. But as a vantage point, it's spectacular. You could use five rolls of film trying to take pictures of the split second when boats plying in all directions make a composition that does justice to the view.
8 p.m. Harbour Kitchen: Dinner in a hotel? The Park Hyatt. That sprawling international box of 1980s brick?
Ah, but the hotel's pristine new Harbour Kitchen looks and feels divine - a calm, modern, self-contained restaurant with a plum waterfront view. Its young chef, Ross Lusted, is one of the hottest names in town, and Sydney Morning Herald restaurant critic Terry Durack has just written a rave review. We attack the deli plate for two, then argue about whether my dinner companion's baked snapper with borlotti bean casserole beats my flounder with roasted pumpkin and salsa verde. My dinner date is an entertaining doctor who swapped Dublin for Sydney 20 years ago and will never be back to stay . . .
10 p.m. Magic Quay: . . . and I think I see why, even after just one day. There are still promenaders, out there on the boardwalk, taking the salty night air. The lights are twinkling around the deep curve of Sydney Cove, turning a working harbour - not one of your plastic tourist-ports - into a shimmering dreamscape again. Don't just take my word for it. "It is so inexpressibly lovely that it makes a man ask himself whether it would not be worth his while to move his household goods to the eastern coast of Australia, in order that he might look at it as long as he can look at anything," Trollope wrote. The good bits haven't changed since 1873.