Piha Beach is famous among Auckland's surfers, and is located only about 40km from the city. Its colour makes it really unique, as the black sand is really cool and unusual. Everyone who visits leaves with a bottle full of sand as a souvenir. Unmissable if you like surfing, and worth visiting even if you don't.
Just past Auckland, heading north on the east coast, you will find the Rodney District, a collection of commuter villages. Further north, the landscape is dominated by beaches, coves, and extraordinary sunrises. If you like sailing, kayaking or hiking, it's a good destination - I know it's a bit far, but it's worth it, particularly if you make a trip in a camper van.