Why not Waianapanapa State Park, Maui?

After navigating the numerous one-lane bridges and cliff-hugging turns of the Hana Highway and then the reaching open green slopes, many visitors are hell bent on continuing to Hana Town. But a better idea is to hang a left on a short detour to Waianapanapa (why-nawpa-nawpa) State Park—among the best in the state, all factors considered. A centerpiece of the park is Black Sand Beach, which has the best snorkeling in this part of Maui, in spite of a near-constant shore break. (If waves are more than a foot or two, use caution, since high wave action creates rip current.) 
The Waianapanapa Caves are reachable via a short loop trail. Mossy caverns create excellent fresh water swimming pools. In the spring, the hatching of tiny shrimp turn the waters red, which, according to legend, occurs to symbolize the blood lost by Princess Popoalaea when her husband (not a good candidate for Dr. Phil) did her in after she was discovered here with her lover. 
A trail—part of the ancient King’s Trail—leads north from Black Sand Beach past the ruins of heiaus and an ancient burial site.
South of the park, the trail skirts Waianapanapa’s rustic cabins (for rent) along lava stacks that do battle with waves. The route continues several miles to Hana, passing the Ohala Heiau along the way.
Final thoughts: 
1. The campground is among the best in Hawaii. 
2. On the road into the park, keep your eyes peeled for honor fruit stands, which alone are a reason to take the side-trip from Hana Highway.
More good stuff to do along the route in your Maui Trailblazer guidebook.