Sydney:
We don't live there anymore but, depending on your interests, I recommend:
A trip across harbour on the ferry to Manly (swimming possible at Manly too).
Back stage tour of the Opera House.
A trip round the city sights on the Sydney Explorer bus.
The Sydney Acquarium is also supposed to be good. I've never been there. Maybe I could hop on a train to Sydney and see some of these sights with you?
When is the planned trip?
I also highy recommend the Great Barrier Reef. I spent 5 days on Heron Island two years ago and the I was really overawed by all the fish and coral. I went snorkelling every day. (The water temperature is much better than Sydney too.)