Diamond Head is one of the many systems of cones, vents, and associated eruption flows which geologists have categorized as the Honolulu Volcanic Series. They were part of eruptions from the now dormant Koʻolau Volcano. Other well-known volcanic landmarks, include the Punchbowl Crater, Hanauma Bay, Koko Head, and Mānana Island.
A nice surprise at Waikiki beach is the statue of Mahatma Ghandi that is placed near Waikiki Beach in the Queen Kapiolani Park near the zoo. I love that he is draped with leis.