Darling Harbour is a fairly busy area of Sydney where you'll find a wide variety of attractions, bars, restaurants, shops, parks, hotels, and a tourist information point, among others. I particularly recommend the Imax 3D Cinema, the Maritime Museum and the Sydney Aquarium. If you want a different experience you can dine in an old boat next to the bridge in front of the bay, or you can take a boat trip to see some beautiful views of the city. You'll also find the monorail station in this area.
Sydney Harbour is one of the most known and famous areas of the city. You can take a lot of great photos of your trip here. Here you will find the Opera House, the Harbour Bridge and the Sydney business area which is full of skyscrapers. It is one of the most touristy parts of the city and there are always people walking around because it is full of restaurants, pubs, cafes and terraces. There are also street performers and musicians such as Aboriginal globetrotters. To see it all with a good perspective you can take a ferry to Circular Quay, where you can see the Opera House and Bridge from a close distance. It is also a great way of seeing the skyline. There are several companies that are dedicated to selling harbor cruises. The Rocks, a neighborhood with old houses full of traditional pubs and restaurants, can be found here, and is one of the oldest areas of Sydney.