Hi, looks like a fantastic trip and you'll have a great time! That routie is a big tourist destinations so you will find lots of like minded people having heaps of fun. If you stay in youth hostels or campgrounds you will definitly find those people 😊
I've been to a few of those places so i'll add what i can about those.
Cairns is a great city and quite big so perhaps try and stay in the main city or check the bus routes if you dont have a car. We've always hired a car here. In the central city are foodhalls, accomodation, tourist places to send you on buses to things, ferry's to islands, pubs, then the fantastic man made foreshore with free lagoon. If youve got 4 weeks then id maybe spend 5 days or so in Cairns as it is a great central location to everything.
We went up to daintree for a day trip one year and it was very beautiful. I wouldnt stay here more than one or two nights as apart from looking at rainforrest, i'm not sure there are actual things to do. But if you do want to look at plants and birds etc then stay longer.
We also did a day trip out to the tablelands. I guess it might be nice to see something of Oz outside of the coast so might be worth a few days again.
Townsville is another biggish town but just really a normal town and in fact a massive milatary town. It has a nice foreshore and magnetic island is literally off the coast and very very close. I wouldnt spend more than a night or two in townsville itself but if you can stay on the island that would be pretty nice i'd imagine (havent actually been to the island myself) 😊
Airlie beach is a very fun backpacker style town. And really the gateway to the islands. You'll have a fabulous time here and youve said a tour so i guess stay for that amount of time - maybe 4 or 5 days???
The next city i've been to is Brisbane and the gold coast. Brisbane is a really nice clean city and you could definitly spend a few nights here. But...if you are going to sydney then sydney will havce everything brisbane has and more - museums, foreshores, beaches, cafes, shopping etc. The gold coast (1-2 hr south of brisbane) might be worth something different for tourism reasons. It is full fo theme parks but if youve been doing heaps of nature things it might be fun to spend a day at movie world. I have also heard that the Australia Zoo is pretty impressive (steve irwins zoo) which i think is 1 hour north of brisbane but i havent been there.
Sydney is amazing, i'd say stay here for at least 3 or 4 nights. You can hang out near the beach and relax or go into the city and enjoy museums, darling habour etc. If you catch the local ferry/transport you can get a cheapish tour of the habour and around the opera house and out to the beach 😊
Have a fabulous time!