When I was still in Port Douglas I booked a tour to Mossman Gorge which is located about 20 minutes north-west of Port Douglas. Cost me $20 just for the return ticket on a bus, life is really expensive north of Cairns.
The gorge was formed by a river that still flows through it today and supplies the towns of Mossman and Port Douglas with freshwater.
There is a walking track about 3-4km long that leads through an impressive wet tropics rainforest.
Some people even took a swim in the more quiet parts of the river. Overall an experience of nature at its best.