EXCITED EMOTICON - just realised that - I think (I'll check) you should be able to go and watch the whales migrating past Nelson Bay - lovely holiday resort which will be very quiet in July/August as you drive up, because it will be far too chilly for most Australians then. They have humpacks and everything!!! They aren't there when we go in March/April, but they pass really close to the coast. You can go out on a sailing boat/catamaran (we recommend 'Imagine' and have dolphins, turtles and whales swimming past you.
We have been to the East coast several times and like Rhubarb would definitely head up north asap. August in Sydney can really be chilly - you could ski in the blue mountains, of course, if that's your thing, but it will bump up your luggage. Sydney Opera house and the harbour bridge, Manley Beach etc will all be windy and chilly. The zoo is good and we had a good visit to a Botany Bay re-enactment museum but most of the attractions are very outdoorsy. (We love the reptile park, which you will pass about an hour north of Sydney if you are heading up the coast. It's small and family friendly - off the main tourist trail, but you can stroke kangaroos and there are good talks for the kids on snakes and spiders).
Ooh, but if you are into Zoos while you are up north do go to Australia Zoo, Steve Irwin's place.
I would love to see Ularu (Ayers Rock) but have never had long enough to take 3 days out of our trip. We had friends go and she hates hates hates flying, but said it was worth it to fly out and stay over for a couple of nights. There is nothing there but desert and the rock, and you do have to be prepared to make a very early start to stumble around for a couple of hours in the dark climb it to see the sun rise, but she loved it.
In terms of stopovers we loved Singapore, and Hong Kong. I know you were thinking of stopping on the way out but we would recommend stopping over on the way home though. On the way out, you are so excited that 21 odd hours just flies by. On the way back you get on the plane and have an 8 hour flight to Singapore, or HK, or KL. If you have a couple of days holiday then, you then only have a 13 hour overnighter to do and you are home - worked for us anyway. We did it once without a stopover coming back and it seemed to last an eternity
Singapore has a great aquarium and zoo (though you will probably visit Sydney aquarium and zoo too) and interesting shopping, though it is hot. Hong Kong is also fascinating and you can travel out of the city if you want to - the bus over the mountains to Repulse Bay and the pretty market at Stanley there is fun.