The super-cute high-tech $13,000 Smart Fortwo, although now only offered in Europe, would be a perfect car for island tourists - it gets up to 71 miles per gallon on a 9 gallon tank and you can park three in a single standard parking space. Note that it only has slightly over half the torque of a Honda Civic, so you don't want to try driving it up steep hills:

Most visitors to Hawaii come in twos and stay on one island. Why not provide an alternative to space-hogging rental cars in Waikiki? And you could fit a lot more of these things on the upcoming Superferry and drive around the islands without needing to refuel: