Road to Hana is great as they all say, go early in the morning if possible. It kinda sucks when the road is flooded with other tourists. And when you go to Hana, continue south down the Piilani Highway....
Piilani is an awesome drive, just for the experience. It's extremely narrow, and dirt in some sections. If you look on the edge of the cliff furthest away in the picture above, you can sort of see the road snaking up the cliff. It's fun, but a slow fun. Remember the road is basically single-lane for some sections so you HAVE TO go slow. As i said some sections are dirt, but trust me any car can make it. We did the trek in a Chevy Aveo! Once you wrap around to the south shore of the island, there's a stretch of freshly paved road that runs along the coast that is probably one of the most scenic drives in Hawaii. It's a shame most people don't see it. I don't know where you heard Piilani is for locals only, but it's a public road that's open to anyone. Rental car companies would rather you not drive it, but as long as you take it easy it's nothing.
And the road up to Haleakala is awesome as well, but as with Hana, there's a lot of tourist traffic. Remember for Haleakala the peak hours are sunrise and sunset, as everyone goes up for those times.