Adelaide is awful, nothing there.
I really liked Brisbane as a city but not much to do for a tourist.
If you’re going to Australia you need to do the Great Barrier Reef (can do that from Cairns) and then as someone else said try do the Whitsundays and/or Fraser Island. It’s be far better than just seeing loads of cities. The cities are like European cities, the unique and stunning parts of Australia are Great Barrier Reef, Whitsundays, Fraser Island and various other coastal areas and beaches. I didn’t actually go but Uluru as well. Personally Uluru didn’t fascinate me though and it takes ages and it’s costly to get to.