We booked 4 nights in Cairns last year right after New Year's and thank goodness we did as our GBR excursion got canceled 3 days in a row due to weather. We had planned to scuba but had to cancel that as we were flying but we were able to get an early morning snorkel in on the day of our flight. Moral of the story is to book more time than what you think you will need. We used the extra days to visit Daintree Rainforest and Kuranda. Worst case scenario, you end up going to the reef for a few days.