About an hour from Milford Sound is a 3 hour hike to Key Summit, a peak that stands at 3,000ft (918m). It's less about the hike (though it is pretty cool) and more about the views at the top. Key Summit is located in such a way that every peak around it is taller, around the full 360-degrees.
These peaks are some of the most dramatic in the country, many of which feature large glaciers and remain above the snow line even in the summer, in particular: Mount Christina 8,116ft (2474m), Mount Crosscut 7,424ft (2263m), Mt Lyttle 6,230ft (1899m) and the lake left behind by their glacier, Lake Marian at 2,280ft (695m).
Key Summit also has a nature conservancy trail around its rim, which is obviously worth it to catch more views, but also to see just how different the vegetation, wildlife and even the colors are in this part of New Zealand. We spent about as long walking around the summit as it took us to hike up to the top.