We did a similar cruise on Diamond Princess last December. The sea days are great for us; we found the 5 port days in a row a bit tiring but I think the 14-day itinerary only hits 4 ports so that may be better.
In Honolulu we used public transportation quite well. We went to Pearly Harbor in the morning and to Waikiki Beach in the afternoon with a short lunch and clothes change back at the ship in between. Worked very well but next time we'd like to take the circle island bus ride; it's the public bus and only $2, ($1 with a Medicare card). I think it runs every hour and takes most of the day. You can get off and visit the Polynesian Cultural Center or other points of interest.
We enjoyed the Botanical Garden tour near Hilo, a Kayaking adventure on Kauai and a whale watch out of Lahaina (which we could have gotten much cheaper right there on the dock).
Had a great time, but then again, we've never had a bad cruise!
Had some rough weather going west and lot of people were surprised about how cool it was the first 2 days out and the last 2 days on the way back, but that was December. It was markedly cooler than southern California because the water temperature is cooler. Those who only had tanktops and shorts stayed inside.
You may want to check out some of the info under the Princess category around that time, Dec 2007. There were several threads about it.
I'd love to do it again! Have a great cruise!