We have just returned from a week staying in a 3 bedroom, beach front apartment (#23 in Mandalay). As a family of 4 with two teens the massive apartment, across the road from the beach was a perfect combination. The huge balcony is completely private with views of the pool and palms / mangrove, it was wonderful sitting reading on the day bed relaxing in the shade.
Everything about the apartment was wonderful, it was spacious, cool, clean and well kitted out with everything we needed - full kitchen, books, toiletries, cards the works.

Being in Port Douglas in this off season was also fabulous, the tours weren't full, we could book the day before and the weather was perfect all week (28-30 degrees)! The stinger net is a 2 minute walk up the beach from Mandalay if you are there in stinger season, we swam there and in the pools every day.
We enjoyed the market in Port Douglas and went to the happy hour at On the Inlet bar for a bucket of prawns to watch the sunset and groper feeding event. Our best meal was at Bistro 3, quite up market with fantastic desserts, we also ate gelato most days as there are many places in the main street. We had a car so drove to other beaches, liked Newell (empty and expansive) and Palm Cove best (had a stinger net and BBQ's/picnic tables beside the beach).
Again in this off season we could wander into any restaurant anytime, quite different than peak as we understand where bookings are required.

Back to the accommodation, there are two BBQ's (we liked the one by the lap pool so the kids could swim while we cooked), two pools and a tennis court. It takes 5 minutes to walk into the main street at our holiday pace. We didn't get there until midnight and had great instructions via email to collect our access keys. There was a huge bowl of fruit in the fridge on arrival. We had nothing booked so wandered into reception for booking advice and bookings on the day, all advice was spot on. We did the Calypso snorkel trip to the reef (I will write a seperate review on this as was so impressed), did the train/sky rail trip and had a day trip to the Daintree taking a crocodile river cruise with Solar Whisper (we saw at least 5 crocs, loads of snakes etc and liked the solar boat with no fumes, great guide). All recommendations were ideal for us, we discussed options and to quickly comment on forums stating resorts push specific tours I did not consider this the case at all with Mandalay the advice was balanced and varied operator wise.

So upshot is, if you are looking for a wonderful relaxing accommodation option in Port Douglas and have children / teens plus want the quiet beach side experience this is the place to stay, we will definately be back to Mandalay. If you are tossing up Port Douglas or Cairns, there is no comparison, Port Douglas is a beautiful beach side town, Cairns isn't as pretty. We also liked Palm Cove and if we didn't have the kids would consider going there as a couple, rather expensive food wise and quite isolated.

For Kiwi's, flying direct Auckland Cairns on Air NZ 777 is fantastic, we did one way Qantas via Sydney and took too long, planes not as good. Great in flight entertainment on Air NZ - so if you can get a decent price strongly recommend.