I have cruised Alaska multiple times on Princess, including on the Dawn Princess.
If you want to do a whale watch I would do that in Juneau. You will have better luck seeing whales there than in Ketchikan or Skagway. Seward is also good for whale watching but you do not say if you are going all that way.
You can wear yourself out with tours that Princess can provide. The helicopter tours are good in Juneau and Skagway so either one would be fine but as I said if you are going on a whale watch do it in Juneau and see what times the tours are than decide where you want the helicopter tour.
You can book tours in advance through Princess online. Pick out the tours you want and see if the times work out for multiple things you may want to do in each port. With the exception of a whale watch I have never done the same thing twice in Alaska and I always find something fun and interesting to do.
Alaska is never the same twice even on back to backs.