I liked Enterprise Car rental. They were at most locations, did not have a cancellation fee, and they had good rates. Tokio Marine recommended getting the collision insurance. With this insurance the car rental company assumes all costs for repair/replacement of vehicle, and it is not reproted to insurance company. Enterprise charged $16.99/day. They also did not charge for extra driver if driver is a spouse.
I wrote notes in a comment somewhere in this trip planner but, can't find it. This is what I remember.
For the cruise, try to reserve 5:30 seatings, this leaves time for shows in the evening. we missed some good shows because we were eating.
However, for the Pride of Hawaii cruise they view the Napali Coast in Kauai at 5:30 Friday night. You want to eat dinner after this, not during. It was beautiful.
Kona and Lahaina don't need rental cars. plenty to do by the pier, unless there is something in particular you want to see/do far away. Nawiliwili also does not need a rental car, but there is so much to see in Kauai. Still want to do the Napali coast and the Hanalei Bay Kayak Tour called the Blue Lagoon through Outfitter's Kauai. They had the best price through TomBarefoot.com. Still want to see the Coffee visitor Center that is on the way to Waimea Canyon. Also, we did not see all views after the last lookout in Waimea Canyon further down the road at Kokee State Park. Need to go next time.
In Nawiliwili the best beginner surfing is right there by the pier in front of the Marriot Hotel. Take the Anchor Cove Shuttle. Near Dukes?
Surfing in Lahaina, Mauui was great for Randle, but crowded. Pretty calm. Freddy and Goto were great teachers and very nice/helpful.
Randle wants to do Snuba, Ellen still needs to make a flower lei.
Hilo may be the best day to hang around the ship, but it is the first day. It is often rainy and the volcanoe is not worth seeing a second time. Lava viewing from the ship should sufficient.
Do not go to Walmart, it is kind of depressing wasting time there, feels like shopping at home. Except, they had the cheapest mauna loa nut/chocolate. bags were 7.99 at walmart, 9.99 at the mauna loa nut factory.