3 week itinerary

Going to Australia for 3 weeks with my other half in September and just wanting some itinerary advice!

I’ve been before but he hasn’t, I’ve seen Melbourne, Sydney and most of the east coast. We’re planning to spend a week in Sydney for a family wedding then travel to Cairns to spend a week there seeing Great Barrier Reef, Whitsundays, maybe daintree rainforest as I loved these and I know he will too. just wondering if it’s worth visiting Darwin for a week (somewhere I haven’t been) or if its better to stay on the east coast or is there somewhere else I’m missing that’s a must see? I don’t mind revisiting some places that I loved but there were parts I’m not desparate to go back to!

any advice on how to spend this 3rd week really appreciated!