If you are interested in a snorkeling trip to the Great Barrier Reef, Wavelength does a really good day trip that includes three locations. It is a smaller group experience with a maximum of 40 - 60 passengers, depending on which boat. They are highly recommended on TripAdvisor, so you may want to check the forums/reviews there.
I was only in Port Douglas for two nights and did not have a car, so the only other activity I did was a short visit to Mossman Gorge, which was a nice scenic walk/hike that took under two hours and was accessible via a private bus from Port Douglas.
Last edited by needtoescape; Feb 26, 2019 at 6:19 am
Reason: typo