This depends. If you will be doing a lot of island activities such as zip lining, ATV tours, Napali Hikes, then it is probably best to have a rental car to get around. However, if you are mostly relaxing at the resort while pondering the meaning to life, dining and shopping next door and walking to the beach at the Sheraton, you certainly can get by without a car.
Additionally, Koloa Landing® has an 11-seat Mercedes-Benz Sprinter and 6-seat passenger van to take guests anywhere in the immediate Poipu Area.