Diamond Head State Monument is the iconic volcanic crater that rises above Waikiki and Honolulu. Formed by an ancient eruption of tuff, the crater is one of Hawaii's most recognizable landmarks and the most visited state monument in the islands. The summit trail climbs from the crater floor through switchbacks, stairways and a tunnel, ending at an overlook with 360-degree views of Honolulu, the coastline and the Pacific. Old military bunkers along the way show the crater's role in coastal defense. Online reservations are required, with a fee of $5 per person and $10 per vehicle for parking. Start early, bring water and wear sun protection, since the trail has very little shade.