Taking the trek up the Diamond Head trail is definitely worth it IMO. Go early in the morning if possible but take water with you. It's not an extremely challenging hike but the trail zig zags as you climb up. As you get to the very top, you'll walk through a long, somewhat narrow tunnel. When you emerge, you'll have the choice of turning right and ascending an intimidatingly steep set of stairs or go left up a less scary route. If you look on the wall in front of you, you'll notice that it's a loop. I would personally take the route to the left and make your way slowly to the top where you can get some great pictures of the area. Then, keep walking down a narrow path to a bunker of sorts where gunners used to guard the coastline. Once you climb into the bunker, walk to the back and you'll find a circular stairwell descending to the lower level which ends up being the top of those intimidating stairs you saw at the beginning of the loop. Walk down those stairs a go back through the tunnel to make your way back to the entrance of the trail. Should take a little over an hour give or take a few minutes depending on how many breaks you take ;)