If you plan on getting outside of Waikiki and Honolulu, renting a car is a must. But renting a Mustang convertible is a sure sign that you're a tourist and practically begs "come burglarize me." A good rule of thumb: When the car is out of your sight even for a moment, it should be empty of anything you care about. But there are other added bonuses of opting out of your Hawaii dream machine—you'll save some money, and won't be hassled by constantly having to put the top down during one of the island's intermittent rain showers.
Reserve your vehicle in advance, especially during the Christmas holidays. This will not only ensure that you get a car, but also that you get the best rates. Kauai has some of the highest gas prices in the Islands.