If it is windy or raining they don't open the rock-climbing wall, but I have seen that happen in the summer, in the spring, in the winter, and the fall.
The goal of the cruise line is to sail with a full ship, which is why the prices drop typically for early December. Bear in mind December is 'Diamond Celebration Month' which means cabins may not last long before being booked. The celebration gives diamond and above loyalty members double points for December cruises. It is why I have 3 of the cruises listed below my name.