There are so many things to see and do it will be hard for you to decide what you should do this time or what to leave for another.
If you will be up at the Gold Coast then make the time to see The Outback Spectacular it is meant to be really good.
If you are going to be spending time in Sydney and can afford it I would recommend climbing the Sydney Harbour Bridge, its an easy climb, we had a few seniors do it when I went up. We had clear skies and it was a beautiful sunset over the Blue Mountains, then a breathtaking view of the city at night. It was really worth the money.
Remember there will be a range in temperatures when you are in Qld, NT and WA compared to Victoria. I think no matter what you end up doing you will have a wonderful time.
I would like to do the Sydney bridge but will it be scary if a little afraid of heights.Its something not to miss kids can do it.Also a harbor cruise.