Kauai is the greenest of the Hawaiian archipelago and just 20% of the island is connected by roads, the rest is wild and can only be viewed by boat or helicopter. Most of the existing roads are paved so that any car can be rented (in general: If you drive with a rental car on an unofficial road/dirt road, you will forfeit your insurance coverage. So read the conditions carefully). Another idea is to rent a scooter, which is also a great way to explore the island.