This is a recommended stop along the road to Hana. The State park features blowholes, sea arches, caves, a black sand beach, and a couple excellent coastal hikes along the remnants of the old King's Highway.
In the 16th century, a chief by the name of Pi'ilani unified east and west Maui to become the first ruler of the entire island. He started a massive construction project called the King's highway, a 4 to 6 feet wide rock paved trail that encircled the entire island. Only a few sections of the King's highway remain including the section at Wai'anapanapa State Park.
From the park you can hike either north or south east along the coast. The northern trail is about a mile one way, ending at the Hana airport runway. The southern trail goes all the way to Hana Bay and is 3.5 miles one way. On this trail you'll pass the Ohala Heiau and several other unmarked archaeological sites (tread lightly).
The black sand beach at the park is interesting to look at but too rocky for comfortable sun bathing. The ocean here tends to be rough with strong currents year round so swimming is not advised. The caves are located nearby the black sand beach.
There's a blowhole on the rocks nearby the sea arch. You'll hear all sorts of hissing sounds as water channels through tunnels under the rocks. It takes really big surf for this blow hole to shoot water. Keep a close eye on children on the rocks here, there are several places, including the blowhole, where they could fall into the ocean.
It's easy to spend many hours at the park if you enjoy hiking. If you're doing a 1 day trip to Hana and your destination is the Kipahulu section of Haleakala National Park (i.e. the Oheo gulch), we recommend you not spend too much time here, and rather spend your hiking time on the Pipiwai Trail at Kipahulu. However, if Hana town is your final destination we recommend you invest more time here..