Sydney is the richest city in Oceania, and by far the most economically important hub in Australia; it is also (by about a million residents) the largest city in Australia. All this wealth and power is on prominent display in Sydney downtown.
The area of the city is quite extensive, and it is useful to break it up into North and South, as the locals do. South CBD includes prominent landmarks such as Central Station, Sydney's Chinatown, and the World Tower, which at 230 metres, is one of Sydney's tallest buildings. At its borders, the Southern CBD shades off into funky Darlinghurst, ugly but busy Darling Harbour, and Redfern/Chippendale, a down-at-hell but rapidly gentrifying precinct. With the exception of Chinatown, of course, there is no real reason to spend a lot of time in this part of town, especially if you won't be around that long.
The real action is in the North, where most of the cultural institutions, shopping and sights are to be found. The big money is centred in the temples of glass, steel and concrete that surround Wynyard and Martin Place train stations. Walking around this area and North towards Circular Quay, the eagle eyed will come across plenty of beautiful buildings, while the cumulative impact can make the region seem like a mini-Manhattan. Meanwhile the best shopping is to be found in the area around the QVB, just up from Town Hall Train Station, where designer stores are to be found, along with bookshops, jewelers and the like.
The most famous sights are to be found in the Circular Quay area - the site of the original 1788 British settlement of Sydney Cove (an earlier, failed attempt at settlement had been made a little down the coast, at Botany Bay – the waterway over which you will have flown upon arrival in Sydney – it's where the airport now is). Circular Quay is almost-but-not-quite spoilt by some hideous architectural decisions, most notably the Cahill Expressway, a highway that runs overhead, and by 'the toaster', a residential development that has filled in much of the open space between the city and the Opera House, and obscured views across to the Botanical Gardens. Almost… but not quite – because the Harbour is irrepressible. The glare of the brilliantly blue water can be hard to take, especially on a sunny day (ie, on most days) so bring sunglasses. That way, you will be able to really feast your eyes on the splendors of Sydney Harbour Bridge and the Opera House.
Most recommended CBD activity? Walk from Circular Quay down through the Botanical Gardens. The views are amazing, and the gardens are beautiful. In the Summer there are evening outdoor cinemas, showing mostly classic films.