Looking out over the Hauraki Gulf from Auckland, you'll find that the central fixture of the horizon is not even be the water, but the looming mass that is Rangitoto Island. A massive, symmetrical volcano, Rangitoto is the youngest in the Auckland volcanic field and proves not only to be a fixture on the horizon, but an accessible adventure destination.
Rising 260 meters (850 feet) above sea level, this dormant volcano is known for its plant diversity and a selection of hiking trails that can be completed in the span of half a day. There are numerous ways to approach the summit, while there are lava caves worth exploring (bring lighting) as a side route, and plenty of trails that skirt the coast of this 5.5 km (3.4 miles) island. Although you won't see molten lava or steam vents, this may be your best bet at seeing the aftermath of volcanic activity in the Auckland area, especially if you're interested in botany. For the rest of you, it's still a cool day trip to an island mountain-top that culminates in some great views of the distant city.
Transport can be had via ferry from the Auckland Ferry Terminal or the Devonport Wharf. Be sure to catch the ferry back, because there aren't any public accommodations on the island.