I have done this cruise three times in the winter. Last year twice - in January and December. Weather tends to be cold heading down to Mexico, but once you land in Puerto Vallarta it typically is warm and sunny. Even to the point of being a little hot during the day. And Cabo being desert can be very warm.
Favorite shore excursions are Jungle Zip Line & Rapelling in Puerto Vallarta - one of the most amazing days ever and not nearly as scary as it sounds. The scenery is incredible, and the entire excursion is a blast. Also the Sea Safari in Puerto Vallarta.
I have never done an excursion in Mazatlan, but when you get off the boat and go out the gate at the dock you are in old town Mazatlan which is really beautiful and going through quite a gentifrication process. If you like history, head into town to see the Cathedral. Old town is definitely a lot more interesting than the Golden Zone.
Favorite excursions in Cabo are the Americas Cup Challenge and I also did a beginners scuba which was a lot of fun.