Well, you sound very confident and I am sure you will have fun, but I do suggest you listen to the person you hire the car from about a recommended route. In oahu, the best thing is to head around Diamond Head from Honolulu and stop at some of the scenic beaches along the way. Just a few miles around the bend you see some beautiful sights.
I am glad it is your first cruise, because it is NCL-America, which is a great way to see Hawaii, but not the best :cruise" experience due to their all-US crew requirement. To get the real cruise experience take another cruise after this, and go on a ship with the typical European staff. Then you will get the feel of a real cruise ship.
Still, you are going to have a blast. Hawaii is great. Which ship are you on?