We took the harbor cruise, though it is not nearly as spectacular as the Sydney Harbor. Thus we would recommend simply purchasing a ferry ticket to Devonport (10 NZD return) and spending 4 hours walking throught the town or and a bus tour of the island. There are good resturants and a two branches of a good chocolate factory.
The University district has a nice park and the Auckland museam is a good place to spend some time if it is quite warm outside. The viaduct at the harbor has resturants and many of the yachts which were in the America Cup race.
There was a listing to our other ports: Christ Church, Picton, Napier and Taurang. There were free shuttles into town for all these ports. Christ church is a beautiful city- the largest on shouth island and worth a full day visit itself. We visited the cathedral, the Avon river, the Museum , which is in the botanical gardens (an excellent Antarctic exhibit and a good cafe to eat lunch). The botanical gardens where the best we saw on the trip.
Picton- rented a car ($65 NZD) from Pegasus rentals and toured the Marlborough Bay wineries in the Blenheim area (30-45 minute drive). Even if you don't drink wine the scenery is worth the visit- reminded us of California, Oregon and Washington wine countries, but much more compact.
Napier- took the 3 hour scenic tour by MountClassictours.co.nz (65 NZD). It includes a couple of nice Hawkes bay wineries and a spectacular climb to 1400 feet mountaintop up the Craggy Hills and a stop for fresh fruit and ice cream.
Tauranga- took Mount Classic Tour 3 1/2 tour of Tauranga, fruit orchards, the Rev. Brown house, and a Kiwi Plantation. The guide was the best we had. He knew almost everything about NZ culture, geography, political system and geology.
Those who don't live by the ocean should take time to visit the beaches, and if you like zoos the Tonga zoo is highly recommended. We enjoyed a couple of hours at the Maritime Museum at Darling Harbor and also the aquarium and wild animal park appeared to be popular attractions. Outside of the city one can take tours of the blue mountains.