The Whitsundays are 74 islands scattered off the Queensland coast, most of them green, steep and empty, and almost all of them inside a national park. The Ngaro people lived and sailed here for at least 9,000 years, in sewn ironbark canoes, until settlers and disease arrived in the 1860s and the survivors were forcibly relocated in 1870. Captain Cook sailed through on 3 June 1770, Whitsun, and gave the passage the name that stuck. Today the Whitsunday Islands are a boat trip from the mainland town of Airlie Beach, or a short flight to Hamilton Island, the one place with a commercial airport.
Here is the honest part. The Whitsundays are beautiful, but they are not cheap, and you can't do them on foot. The mainland has no good sea beach, because from about October to May the water is open to box jellyfish and Irukandji. Everything on the postcards, Whitehaven Beach, Hill Inlet, the outer reef and Heart Reef, needs a boat or a plane. Three days is the right length: one on land, one at Whitehaven, one on the outer reef. Fewer and you skip the beach or the reef; more and you are paying for a second boat trip.
Day one stays in Airlie Beach, with the Conway National Park walk, the free lagoon and the foreshore. Day two is the Whitehaven and Hill Inlet boat day, with the overnight sailing option explained. Day three is the outer reef at Hardy Reef and Heart Reef from the air. Hamilton Island is covered as the alternative base, and in the "more time" section as a day trip. Start on a Saturday so the foreshore market falls on day one, and book both boat days before you land. This is also a corner of Australia where November is the start of stinger and cyclone season, which is why there is a section on both. Going further north? See 5 Days in Cairns for the other half of the Great Barrier Reef.