Two big advantages to driving to Milford sound: 1. you can get there before the bus and it is magical when devoid of the tourist hordes (esp. if fogged in, which is likely for March). 2. you can stop to see some great sights along the way (things like bubbling brooks with stunning rock formations) ... it is an enchanted part of the world. As for the rest of the time, possible suggestions are to stay in Arrowtown (former gold mining town near ZQN) and to drive to the glaciers (though maybe not enough time for that). The glacier walk is maybe the best single tourist experience that I have had in the past 20 years (but you have to be reasonably fit, you are equipped with crampons and ice axe and need it for the glacier).