The World War I gunnery site on Diamond Head crater is slowly falling to ruin, now. It is not easy to discern locations that we saw on the show. Of course, fifty years have passed since then, and the battery was fifty years old, then! Battery Harlow appeared in several episodes of Hawaii Five-0. The first time we saw it was in “The Ransom” (Season 3), where it was the hideout used by the kidnappers to hold Timmy Blake and Kono. It was also the site to which McGarrett & Co. were led to "watch the birdie" in "Fools Die Twice" (Season 5).
Battery Harlow was where McGarrett and Intelligence met in "The Ninety-Second War" (Season 4).
It was the hideout of the People's Attack Group in "The Young Assassins" (Season 7). See photo: http://www.diamondheadhike.org/harlow/imgp5660.html
Finally, it was where Officer Sandi Welles (Amanda McBroom) and McGarrett hid from Kim Chee's henchmen in "Loose Ends Get Hit" (Season 8). See photo: http://www.diamondheadhike.org/harlow/imgp5670.html.
Smaller batteries dotted the landscape at Diamond Head. We saw one in "And I Want Some Candy and a Gun the Shoots" (Season 5) and "Little Girl Blue" (Season 5).
Note the bunker on the left, where McGarrett and the HPD scaled the ocean side of Diamond Head. The turn-around where the cars are parked is where the helicopter landed. Diamond Head Lighthouse on the right remains just out of view in the episodes.