Gordon River cruises are Strahan's headline trip. Boats leave the waterfront, cross Macquarie Harbour, pass through Hells Gates, stop at Sarah Island, and continue into the lower Gordon River, where ancient rainforest grows right down to the black water.
It is a long half-day, but a good one. The cruise gives you the scale of the harbour, the isolation of the old convict settlement, and the strange calm of the river after the windier open water.