Puʻu o Mahuka sits on the bluff above Waimea Bay, and it is the largest heiau on Oʻahu — close to two acres of stacked stone laid out as three walled enclosures, with walls running from roughly three to six feet high. The name is usually given as "hill of escape". It was probably built in the 1600s, it is a National Historic Landmark, and it is a sacred place that Native Hawaiians still use.
That last clause is the one to hold on to. This is not a ruin in the European sense, abandoned and available. A heiau is a temple, and this one was a major one — a luakini, associated in tradition with the highest level of Hawaiian religious practice and with the kāhuna who served the chiefs of this district. Tradition connects it to the kahuna nui Kaʻōpulupulu, and to an incident in 1794 in which members of a British ship's crew were killed at Waimea and, by some accounts, brought here. Offerings are still left at the site by people for whom this is a living practice rather than a photograph.
What a visitor sees is deliberately undramatic: a long, low platform of stone on an exposed bluff, marked paths around it, interpretive signs, and a considerable view. From the edge you look down over Waimea Bay, along the reef line towards Haleʻiwa, and on a clear day out to the northwestern tip of the island at Kaʻena. It is the only place on the North Shore where the geography of the whole coast resolves into one picture.
Practically, it is free, it is a State Historic Site with posted gate hours, and it is reached by a narrow climbing road that most rental cars manage and no large vehicle should attempt. It takes twenty minutes to see and it is five minutes from Shark's Cove.
The rules are short. Stay on the marked paths, do not walk on or into the enclosure, do not move stones, and do not leave offerings.