Jen127, if you're going in July next year then prepare for it to be cold! July in Aus is the height of winter, particularly in Sydney!.
Port Douglas is lovely but you might find it a bit quieter as it's not peak time. A trip to the Great Barrier Reef and there are companies that will take you out if you're not a strong swimmer/non-swimmer. If you can, a trip into the rainforest with an aboriginal guide is well worth it, the wealth of knowledge of the plants and animals is incredible - not to put you off, but watch out for the golden orb spider webs. You'll understand if you see one. Not sure what your budget is but try the Thala beach lodge - just outside Port Douglas and if you want to get close up and personal with the wildlife then you can't beat it! We had stick insects (a foot long) and frogs on the balcony, birds of all descriptions flit through the trees and frogs and toads everywhere. Oh and when you've had enough, shut the doors, put the air con on and sink into a luxury bath filled with yummy smelling products.
Brissy - South bank with a bottle of white wine at night, a trip on the CityCat (boat trip) at night to see the city lit up. It's just the standard commuter boat but well worth it. Mount Cootha has a lovely cafe at the top and a fabulous view for miles over the Glasshouse mountains to the North.
If you've got time, a trip to Noosa for a bit of sun is lovely and the walk around the headland gives you wonderful views, especially if you go first thing and then get a fab brekky on the way back down - 1hr drive north of Bris.
The Gold Coast and Gold Coast hinterland (45mins drive south) are well worth a visit - the Gold Coast is the home of Surfer's Paradise so essentially a beach that's miles long but very bright lights and glitzy hotels. Gold Coast hinterland is worth exploring by car. There's a treetop walkway about 40mins drive south of Brisbane that's worth a look as it explains all about the rainforest. Oh and thinking about it, a trip to the koala sanctuary to the west of Brisbane means that you can come back with the all important pics of koala cuddling (just don't call them bears........)
Sydney - I guess all the usual things, Sydney Opera house, the bridge, the Rocks. The Lord Nelson pub has v good accommodation and fantastic pork pies! If you can, escaping to the Blue Mountains for a night is a really good idea. There are fantastic view points and I would highly recommend Lilianfels hotel.
God, I really miss Aus sometimes :( 
Have a fantastic time!!!!!