I second taking the full route when doing to the Road to Hana. Most people either stop at Hana or 'Ohe' O Gulch. And yes, rental companies might not allow you to take the car further because the roads are unpaved and treacherous for the first several miles. You get past that though, you hit freshly paved road and some of the most unspoiled terrain you will ever encounter. And hardly any other humans. That was one of my favorite moments from Maui.
