I love Collins Street, the shopping and atmosphere are great. Australia on Collins is the best place to start, the shopping center stretches between city blocks. There are heaps of up market shops in the central city blocks, heading south on Collins takes you to a business district that is dominated by large corporates like the ASX. You should explore a bit though because the Rialto is on Collins and has a nice observation deck.

There are plenty of little side streets so make sure you explore the shops and small streets the break off Collins Street, you will find the best food and the nicest people and you're likely to discover a really unique shopping experience.