Can anyone advise what i can do in Port Douglas just booked a Luxury stay for me and my girlfriend with a swim up bar

i don't mind if it's not train related.
Definitely do a couple of reef trips- one inner & one outer reef at least but choose your operator & day for weather conditions carefully. Do good research before you go don't just chance it at PD dock on the day as the good operations fill up well in advance.
Spending a bit extra on this once in a lifetime experience is really worth it.
Avoid all large vessels- you want no more than 15-20 on board & if just snorkling choose a snorkle only operator. Those that mix divers & snorklers are a waste of time as you just sit around for ages waiting for the divers to kit up.
The good operators also split ability groups- there is a big problem with Asian tourists saying they can swim when they can't & then ruining the trip for others by holding the group back.
Also get some good sea sickness medication - ideally purchased on your Asia stopover as the stuff sold in the EU is rubbish.
A great little trip is Granite Gorge rock wallaby park. Its run by a lovely but batty old hippy & is about an hour from PD & is wonderful. There is a huge colony of rock wallabies who you can feed and there is some good walking amongst the granite outcrops.
As has previously been said Daintree is well worth a visit though leave v early or go late in the day to avoid queues at the car ferry.
I personally found nothing to like about Cairns at all & other than using it as a transport hub I wouldn't waste your time.
The gorge railway is one that I chose to avoid. It is massively overpriced, too hot and packed with tour groups from the cruise ships. You can visit the area anyway in your rental car and get better views at your leisure.
Don't forget to have a ride on the sugar cane steam train which runs in PD on Sundays (though check their website if that has changed)
Definitely rent a car whilst you are there to explore though I strongly advise taking out an annual excess cover policy in the UK(£30 or so) as rental excess in Aus are v high due to the number of uninsured drivers plus roo strikes- though they are less of an issue in that area than say WA.
The best bits of advice I was given in Aus was don't drive at dusk (as the roos are most active) & if you have to, tuck yourself up behind a truck or tour bus as they will hit the roo not you!
Whatever you do don't swerve to avoid hitting one. Sadly there are many incidents where a tourist has swerved and then hit a very hard and going nowhere tree.
One QL also said you will never see a local swimming in any river or rock pool however inviting. Think you can work out why!