I did a 10 day trip around the south island just before a long OS stint for work, and you are in for an awesome time! Some places I went FYI:
Nelson - beautiful little town. Walked 4 hours on the Abel Tasman national palk walk, and the path was gorgeous. Seal colony is worth seeing, as some of the other temperate rainforest around that area. Lots of cottage industries around here as well.
Franz Josef Glacier - one of two places in the world where a glacier runs through a rainforest, as I understand it. Pretty spectactular! There is some great lake kayaking around there as well. The coastline from Greymouth down to there is beautiful.
Wanaka/Queenstown - great skiing towns and nightlife is pretty lively.
Plus lots of great little towns on the way. Stay in hostels rather than hotels, they are much more pleasant and you meet more people.
I didn't get to Milford sound, but everyone I met raved about it... definitely go there if you plan on staying around the south of the south island rather than the north.
Have fun!