We have 4 nights in Port Douglas.
Day 1: Arriving in Cairns ca 9.15 am, driving up to Port Douglas, should arrive 12-1 pm.
Day 2: Nothing planned.
Day 3: Booked a day trip out to the Reef.
Day 4: Nothing planned.
Day 5: Have to be back in Cairns no later than 4 pm.

Want to experience Mossman Gorge, Daintree Discovery centre, Cow Bay and Cape Trib by car. But as the wife wants a day to relax is this all feasible in one day or is it better to do everything during one day but leave eg Mossman Gorge until the last day before heading back to Cairns for 4 pm?

I think leaving something for your departure day makes sense. Sometimes the last day of a trip can feel like a lot of hanging around, waiting for a flight. Just check you'll have enough time for your last day activity. Leave plenty of time to drive back to Cairns.

What time of year will you be in the far north? Hopefully our winter, as summer can pose problems with weather.

Perfect, then Momman on the departure day.
We are arriving in Cairns 19th July, dropping off the car at 4 pm 23rd and then leaving Cairns on 25th by train.

Mossman, not Momman....

I did a Trip Report a few years ago when we travelled by train Cairns to Brisbane and also Townsville to Cairns. I'll try and find the link in case of interest.

Thanks, but we are just travelling by train from Cairns to Townsville where we are heading straight out to Magnetic Is and then we will travel down to Airlie Beach 4 days later when we will head out to Hamilton Is for 3 days before we fly to Sydney.