Welcome to Cruise Critic!
First, there is a board here devoted to Hawaii, located here:
https://boards.cruisecritic.com/forum/37-hawaii/
I would go over there and start researching what you can do on your own. It is pretty easy to get around on your own, so no need to pay for a private excursion.
Viator is an broker, they don't provide the tours themselves. They take your money, and contract with a low bidder contractor. Yes, this can give you some good prices. But if you have any difficulties, it can be very hard to contact someone who can fix the problem for you. Rather, it is very commonly recommended that you deal directly with recommended local tour vendors.
Lastly, there is a board devoted to Princess here that will get you all the Princess info you will ever need:
https://boards.cruisecritic.com/forum/119-princess-cruises/