The legendary “Road to Hana” drive seems to indicate that the town of Hana itself is the goal, but you'd be crazy to miss a visit to Wai'anapanapa State Park.
Spending some time in Wai'anapanapa State Park is reason enough to stay overnight in Hana. It's a lush and gorgeous park just outside of Hana, and one of its most well-known features is the small black sand beach of Pa'iloa. It's a beautiful beach, to be sure, lovely for swimming or simply sunbathing, but there's more to this park than just a beach.
Wai'anapanapa has two underwater caves you can visit that are filled with a combination of fresh and salt water. You can go swimming in these pools, too. This area also has historical significance, too, as you'll see when you visit the ancient burial sites. There is also a trail that winds three miles along the coast, from the park all the way into Hana Town itself.
Wai'anapanapa State Park is located at mile marker 32, just before you get to Hana. There's no entry fee to get into the park, and parking is also free. There are camping facilities, including restrooms, drinking water, and outdoor showers, so you can spend the night here before heading back on the Road to Hana.