Wai'anapanapa State Park is a highly recommended stop along the Hana highway, but note that you must make
reservations in advance to visit the park! The park features blowholes, sea arches, a black sand beach, and two 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 see, but not really a beach you would typically want to swim at. The ocean here tends to be rough, with strong currents year-round and swimming is not advised.
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.