A must-see at the Great Barrier Reef is the Heart Reef, though I must say its smaller than expected (third photograph, center). 

A personal recommendation: Whitehaven beach is paradise, and my photos simply don't do it justice. Accessible only by seaplanes that drop you beachfront, your feet will melt into the fine, fine sand.  (Full disclosure: I took some home with me). Make sure your seaplane returns to pick you up, however! There's no other way off the island, and no known inhabitants...