Kahanu National Tropical Botanical Garden is the home of Pi'ilanihale, the largest ancient temple in Hawaii. This is a recommended stop if you're in time to visit and don't mind paying the $10 per person fee for adults 13 and older. The garden walk is only half a mile and can be done in half an hour, although you'd probably want to spend a little more time here. The coastal views from here are lovely, but the main attraction is the temple remains, not the garden.
Archaeologists believe Pi'ilanihale Heiau was built in several stages, possibly starting in the 13th century with major construction commencing in the 16th century, coinciding with the rule of the Pi'ilani family of chiefs. Pi'ilanihale translates to house of Pi'ilani.
Although Pi'ilanihale Heiau is the largest and most impressive temple in Hawaii, it's omitted from Hawaiian Oral History. It is thought that talking about the temple was made a Kapu in the 17th century, following a bitter rivalry between the two sons of King Pi'ilani. One of the sons eventually vanquished the other, and put a Kapu on any talk of his brother, or the home he used to live in, which was the Temple. Hawaiians living in the area confirm that their parents and grandparents forbid them to approach the area.
As such, the temple was consumed by the Jungle and mostly forgotten until the 1960s, when an archeological expedition cleared the area and the site became a national historic landmark.
The garden is open 10am to 2pm Monday through Friday. Even if you arrive just before 2pm, you'll be allowed to explore until 3..