An overnight cruise on the Doubtful Sound should not be missed. Outfitter Real Journeys, which has a long and respected history in New Zealand, does a fantastic job of escorting gobsmacked visitors through this incredible waterway.
The boats make their way out into the sound, generally stopping in the afternoon for water activities—travelers can choose to kayak or take a tender boat with a nature guide, or even to swim, if it's warm enough. Trained naturalists point out geological wonders and wildlife that you might encounter—penguins, seals, and dolphins are often around.
The private cabins are modest but comfortable, and you'll want to spend all of your time on deck, anyway. Meals are served buffet-style and there's a cash bar with wine, beer, and liquor.