I've never sailed with Carnival so don't have any idea of what they offer. I've also never been to Hawaii but I am booked on a cruise which leaves in February. Here are some of the sites I've been using to plan our trip. In addition, plug in the name of each island into your search engine and you'll get too much info.
Hawaii is the perfect place to NOT take a ship's tour. Get some guidebooks before you go, and rent a car. You can get away from the normal tourist crowds this way.
For example, in Honolulu, I would skip Waikiki unless all you want to do is shop and drive to the North Shore (in winter this is where the big surf is), swim at Ala Moana beach and go to Pearl Harbor and Punchbowl Cemetary on your own.
In Kauai, drive to Waimea Canyon (south shore) or all the way to the end of the road at on the north shore to see some beautiful scenery. Stop and have a shave ice if you are hot!
In Kona you can walk around town, or take snorkeling trip (highly recommended). In Hilo, you can drive yourself to Volcano National Park, to the Hilo Gardens or the the Falls unless you want to go with a group.
In Lahaina, shop all day, or get a car and drive north or south of town for more secluded beaches. With a long day you can get to Hana and back.
You can stop when you want, see what you want and if you have a party of two or more it will be cheaper than an excursion.