I have another suggestion for those of you who are in search of something that is not on every tourist's itinerary. Take this with a grain of salt, as I can't vouch for it, because I have never been, but many of my coworkers and friends who have visited New Zealand actually preferred Doubtful Sound to Milford Sound.
Though not as famous as Milford, Doubtful is nevertheless exquisite, and peaceful to boot. Because of this, you are more likely to see bottlenose and dusky dolphins, fur seals, as well as the Fiordland crested penguin in October and November. Trips to Doubtful Sound can be obtained from Te Anau, the same place that is the starting point for the Milford Sound cruises. A cruise can also be combined with, of all things, a visit to a power station, whose machine hall is 2km, or more than a mile, underground.