Milford Sound was awesome, and probably the climax of the trip, but the Tongariro Crossing hike was a serious rival.  Called "the greatest one day walk in New Zealand" it is 10 miles through lava boulder fields, up and over a volcano, through the crater, past steam vents, Emerald Lakes, and down through red tussock grassland with views of mountain lakes.  This was a multi-sensory experience.  We busted our asses going up "The Devil's Staircase", froze our asses on the Red Crater summit, saw landforms that looked like mile high abstract paintings, and smelled the sulfur bubbling from the earth.  What a way to spend our last day.