If there is one geological formation that represents the island of Oahu, and perhaps the entire state of Hawaii, it is Diamond Head. This crater, formed about 300,000 years ago, prominently sits right next to Waikiki and makes its way into many skyline photos of Honolulu.
Used for the military as part of the Oahu coastal defense system, there are a variety of tunnels and trails in and around the crater walls. While Diamond Head is still used by the military, one of the paths up the inside of the crater is open for tourists.
Cost for entering is $5 per car, or visitors can park outside the crater for free, walk up the road into the crater, and pay only $1 per person.
The view from the top is worth the short but steep hike including some steps and dark tunnels.
The best time to visit Diamond Head is during the week, as the trails are quite crowded on weekends.