Could have easily spent hours wandering through the old military tunnels and grounds, which date from 1885. The DOC historic site has awesome views of the harbour, it’s not far from Devonport in Auckland, and is open to the public daily, for free!
One of the highlights has to be the disappearing gun (as seen below painted green), named because it vanished under the ground for reloading after each shot (the slow speed of this process was its downfall). Definitely recommend North Head as an interesting place to visit with heaps of sign posted history along the way!
Forgotten NZ home page for more places to discover in New Zealand