Australia is a continent-country: the Opera House and Harbour Bridge in Sydney, the coffee and street art of Melbourne, the Great Barrier Reef from Cairns and the red desert of Uluru. Beaches, koalas and vibrant cities in a single trip.
Given the distances, plan 10 to 14 days and combine domestic flights: 3-4 days per city works great. Tell the AI how many days you have and it builds the plan in stages, with an estimated budget in Australian dollars.