A little research might lead you to one of the few Big Island companies allowing visitors to drive on Saddle Road and to the summit of Mauna Kea. However there is some fuzziness about how much off-roading you can do with these island car rental. We can promise you they are all very strict about damage and will check the car very thoroughly, including with mirrors under the car, when you return it. If you are on any other island, it doesn’t matter who you rent from or what you rent (including a Hummer). If you off-road, you are technically violating your rental agreement.