Love this park. Trails (paved walkways and stairs) are very very easy. The spring is fun to jump into but very cold. If you go into the back of the left cave in spring you use to be able to climb it (in the dark) and go through hole to surface. Fun but kinda sketchy.
Definitely take the hike beyond the black sand beach. The majority of tourists don't go beyond the beach and the views are wonderful. The bright green plants on the black rocks will make you feel like you are on the set of a 1970s sci fi tv program.
Very quick hike to a set of fresh water caves. The trail is slick, but well maintained. This is a very popular attraction, but is not easily accessible for strollers and young ones, so the crowd is older and a little rowdy. One of the fresh water caves is a popular spot for diving into the water.
From Pailoa Black Sand Beach (and really cool caves) at Waianapanapa State Park you follow a portion of the ancient Hawaiian Kings Highway from Pailoa Bay to Pakaulua Point. The trail goes along the shoreline to the left of the beach, and was in need of trail maintenance when we went. The trail ascends black lava formations to the Point. The scenery is unreal and the breezes are great. Watch out for woody underbrush and be careful along the sharp lava. My shoelace got caught in a bush and I tripped against the lava - result hematoma and cut on my shin... cleaned and cooled off in the healing waves of the black sand beach - this is an awesome place!