Great Keppel Island lies just North of the Tropic of Capricorn. The nearest main centre on the mainland is Rockhampton, more than 600 kilometres North of Brisbane. You can take the ferry to Great Keppel Island from Yeppoon, there's one several times per day.
It has all (or most) what you expect from a tropical island, including opportunities to snorkle, secluded beaches, bushland and the odd resort. The nice thing about the island is that it's not over-developed and not too big. The climate is warm and sunny, all through the year!
And did we mention the beautiful sunsets?