We were in Cairns last September and it was great!
One "must do" is definitely the Kuranda Sky rail and train like Suse said, it's fantastic and there's the Kuranda markets when you get up there and the butterfly sanctuary is also there which is great, DS was fascinated by them!
The daintree is also a couple of hours up the road which was nice, there's croc cruises which were good too!
Hope you have a great time!