Milford Sound is always worth it, although the overnight cruise can be expensive. The sound can be visited as a day trip from Queenstown, but it is a very long day trip (still stunning, though). If you have your own car you can go at your own pace and stop the night at Te Anau and combine a visit to the sound with a day walk in the vicinity. There are a number of day hikes around Wanaka (near Queenstown). Mount Aspiring National Park (accessible from Wanaka) has a marvelous hike that takes you to the rims of a glacier peeking over the mountain. There are other hikes right along the lake that gives you wonderful views and you can decide how high you want to ascend.
Mount Cook is an absolute must and there's a lovely hike (if it's not raining) that gives you wonderful views. Between Mount Cook and Lake Tekapu is a huge sheep station called Glenmore and they rent out rooms in their lodge and we stayed there for a night and saw the most impressive display of the Milky Way I have ever seen (there were so many stars that the sky literally glowed). Near Glenore is an observatory that is open to the public and has a cafe attached to it. Views are lovely and it is indeed very windy.
Castle Hill (near Arthur's Pass) is an amazing cluster of rock formations that resemble the ruins of castles of ancient giants. Kids love exploring the rocks and running up and down the hills. Near Castle Hill is an underground stream called Cave Stream that is navigable on foot (be prepared to get your feet wet, so strong sandals are a must, as is a flashlight). Think the Goonies

It's about 45 minutes to walk through the underground cavern from one end to the other.
How many days is your trip?