Diamond Head, called Le'ahi by the native Hawaiians, is one of the obvious landmarks near Waikiki. According to the notes they had navigational fires on the summits along with a temple to the wind god to protect the fires from strong updrafts. Today there is a lighthouse at the base of the makai (seaward) slope of Diamond Head.
In the early 1900's the military created a Fire Control Station in the rim of the crater. The path hikers use was mostly created then. Most of the monument is still controlled by the federal government and is restricted access.
Here's a map showing the overlapping areas. The green area is the monument. The blue area is the park or public access area. The park and the road in were opened to the public in 1973.
The area inside the crater is drier then the areas outside the crater and there are some birds and other life to watch for on the trail. There are several signs at the Information Booth next to the parking lot. One critter I didn't see mentioned are the tiny mice that I saw several times along the trail.
Shortly before the first flight of stairs is a small platform holding the rusting remains of a winch and cable system that was used in the 40's to hoist machinery part way up the crater wall to the Fire Control Station.
After the first flight of 74 steps you'll come to a tunnel carved into the crater wall that is supposedly 6'1" tall or maybe a little bit taller. On my way in I didn't see the sign showing the height. I saw it on my way out.
At this point you get to chose between climbing the 99 steps to your right which have overhead pylons to hold camouflage netting.
Or you can go around to your left and climb the outside of the rim.
It offered a respite from stairs so I went around that way. This is one area where you can get some extraordinary views of Koko Head and Fort Ruger mauka (inland) from Diamond Head.
After taking in the views from this level you're facing more steps, metal ones this time, to get to the top of the Fire Control Station.
This leads to a path near the summit from which you can climb to the summit or enter the Fire Control Station.
I suggest going up to the summit for a spectacular 360 degree view of the area.
As you can see from the middle picture there are retired bunkers/gun emplacements visible around the rim of the crater. From here the observers could see ships well out to sea and the artillery could be easily directed. A note I saw there indicated that the artillery in the crater was capable of hitting Pearl Harbor on the far side of Honolulu.
At the top was a park ranger handing out tokens as souvenirs and offering a certificate indicating that you reached the summit of Diamond Head.
The other thing to see at the summit is an info panel with a map of the area and a compass indicating the directions to several sites of interest. Including Australia some ~3800 miles away.
After this head back down go through the inside of the observation area. You'll have to crawl to get in, but it is an interesting room with a flat slit giving folks in the room a pretty good view of the ocean in front of them.
The man in that picture will have to stay bent over like that for about another full step to get outside.
From here you go down the spiral stair, perhaps taking the time to look into each level of the station.
And then out a tunnel to the top of the 99 steps I didn't take earlier.
The walk down is an easier hike than the hike up, so expect it to go more quickly. In several places there are signs warning hikers to stay on the trail to prevent uncontrolled erosion. Often, as in the case below, going off the trail would probably end in broken bones as well as erosion from tumbling several hundred feet down the steep, rocky crater wall.
Diamond Head is a reasonable hike for anyone that is in okay shape even if they aren't athletic at all. Having said that it is pretty much wall to wall tourists so you shouldn't think of this as a wilderness hike. You can either take a car in for $5 or walk in $1 a person. It is an easyish walk from Waikiki and a manageable walk from Ala Moana. If you prefer you can also take a bus to the entry point of the state monument and walk through the Kahala Tunnel. Do remember that the hikers aren't permitted to start for the summit after 4:30 PM and really you should plan on getting done and out of the park not much later than 5:30 PM, including 2 hours to climb to the top and then back down. If somehow you don't make it down and out by 6 PM you could get locked in and be in real trouble. The gate at the entrance to the Kahala tunnel is closed each day at 6 PM.