Should travellers add Adelaide, Kangaroo Island or South Australia?
South Australia is worth adding when food, wine, wildlife, coast or a quieter city rhythm fits the trip. It is less convincing as another stop tacked onto an already crowded Sydney–Melbourne–Queensland route.
Answer
Adelaide is the easiest way to add South Australia when food, wine and a quieter city rhythm are the point. Kangaroo Island is a bigger commitment, best when wildlife and coast matter enough to justify the extra time rather than becoming another stop squeezed between cities. A broader South Australia leg makes sense when you want wine regions, coast and slower road-trip days; it is less convincing when it is being bolted onto an already crowded Sydney–Melbourne–Queensland route. If the route is already full, save South Australia for another trip rather than skim it.