If you don't mind stretching your legs...Train to Museum station. Walk through Hyde Park, then out past the Art Gallery of NSW to Mrs Macquarie's Chair, you'll get a great postcard view of the Opera House and Harbour Bridge. From there walk through the Botanic Gardens to the Opera House, around Circular Quay, check out the Rocks and then (depending on time) walk across the Harbour Bridge or head to the Lord Nelson.