The Routeburn Track really is stunning. The cloud closed in on the middle day from Harris Saddle to Lake Mackenzie, but the views across and down the Hollyford Valley on the last day while walking past Earland Falls to Lake Howden and the Divide were beautiful. The relatively high huts (compared to e.g. the valley huts on the Milford Track) had some great short side trips close by.
The red beech forest surrounding Lake Mackenzie is also spectacular, with a mind-boggling palette of different greens.