It really depends what you are looking for in your trip, as the entire QLD coastline is spectacular, and each area you have mentioned offers something totally different to each other. With the theme parks this is it in a nutshell: Sea World (animals, several animal shows, but limited rides, good for the kids but not for the thrill seekers), Movie World (great place for the young at heart with fantastic rides, not so good for young kids and the elderly), Dream World (similar to Movie World). I'd honestly give it a miss as being from Singapore you have access to Universal Studios, the dolphins at Sentosa Beach and the Zoo. The Gold Coast also has the Tamborine Mountains if you are looking for scenic, not to mention the endless beaches. Sunshine Coast is very relaxed, about a 2 to 3 hour drive from the Gold Coast. It is scenic and very cosmopolitan. The highlights are the Underwater Aquarium, Australia Zoo and the markets. I would recommend Caloundra as its in the southern most point of the Sunshine Coast, and is quite stunning with the walk and beaches. You would need to fly the leg from Gold Coast to Cairns, its a VERY long drive. The Esplanade walk is one f the most stunning walks along the ocean. There is also the Kuranda Scenic Railway and Skyrail, Great Barrier Reef day trips, Cape Tribulation/Daintree tours, Green and Fitzroy Islands. The nightlife in Cairns is also world class and the culinary experience is second to none.