Rented car was there for 2 days free parking overnight by ship. Maui was great, not much to do by the ship. BTW we took a helicopter tour with Blue Hawaiian which we booked ourselves which was truly spectacular.
Rented car for 2 days in port, probably could have parked on the street overnight for free but paid $20 to park in a lot. Kauai is spectacular do not miss Wiamea canyon. Also although the port where the ship docks is a little industrial looking there is a very very nice beach in front of the Marriott that you can walk or take a free shuttle to. The rent all sorts of beach equipment plus Marriott has a lot of chairs and conveniently uses the same color towels as Norwegian.