First time to Hawaii?
Hilo, don't miss touring the volcano. You might be best off here to take a tour, rather than renting a car. Getting to the volcano is kinda tricky (well, not to the park itself, but finding an active flow) Here is what I would suggest. Take a 1/2 day tour to the Volcano and then take the walk from the port into Hilo that is described at http://www.hawaiiportreviews.com/HiloHawaii.htm
This will give you the best of your visit to Hilo.
In Kona, I am not sure that I would rent a car, unless you had something specific that you wanted to see. The distances are great getting around the Big Island and you can burn a lot of time just driving somewhere. I think I would tender into Kailua, explore the town and then partake in some activity such as snorkeling, fishing, beaching it up or enjoying one of the many excellent restaurants in Kailua.
Ditto on Maui. Are you tendering into Lahaina, or docking in Kahului? If you are in Kahului, then by all means do rent a car (airport is only 5 minutes away) and go to Lahaina and Kaanapali Beach. If you are tendering into Lahaina, I would just spend the day exploring Lahaina and then take the shuttle out to Kaanapali. Again, unless there is something specific you really want to see.
Nawiliwili is basically a commercial harbor for Kauai and this is one island I might consider renting a car to either go visit Waimea Canyon or trek out to the north shore and Hanalei. If you wanted to stay around Nawiliwili, visit the Mariott Hotel. You can set up on the beach and drink and dine at Dukes on the Beach. It is a great beach day. If you want the best beach on Kauai, then rent a car and drive to Poipu Beach.
Oahu is easy. You can take the bus everywhere and avoid massive traffic and parking fees. The busses are frequent and popular.
for more ideas. There is a ton of things for a first time visitor to Hawaii to do without renting a car, as Hawaii is an enchanting destination and you won't have to go looking for it, it will find you, no matter where you are.
Have a great cruise!