New Zealand -South Island

I would skip the train. Time better spent driving. Not to miss are Milford sound (we did an overnight and loved the experience). Glaciers and surrounding areas are a must. Go take a look at the bungy jumping bridge outside Queenstown. You won't believe the color of the water.

You might consider touring Doubtful Sound in addition to or instead of Milford Sound. We found it to be more scenic and not as crowded.
Plan on lots of time to drive. Almost all roads are 2 lane and the maximum speed is 100 (60 mph). Bridges are one lane and you often wait for opposing traffic. There are very few bypasses around towns. The driving is scenic but not fast.
We took the ferry from Wellington to the South Island. There were whitecaps in the harbor when we left and as we got into the channel the trip was very rough. We could not see any wildlife or scenery until we approached South Island. In calm weather it would be beautiful. We were there in mid March 2012.

If you like seeing glaciers and mountains, you might want to consider spending some time based around Queenstown and if possible a flight over Mt Cook, to the west coast, over the glaciers. Queenstown is a real hub for everything adventurous and beautiful scenery surrounding it. I would concentrate my research on it and travelling around there. The south is great for bush walking ( hiking)
Towns like Invercargill and Dunedin are quaint, but for memory are not 'must see'. given the driving required. You should not have too much trouble with traffic as the south island is sparsely populated. The scenery is spectacular, especially when you get up high on the mountains and look down over the surrounding fields etc. Think of a 'compressed' Europe. Where as my experience with the North Island has been as less dramatic scenery, overall.

Hope you have a great time.

Carolyn, we spent 10 days in Queenstown a few years ago during the month of July. We did the MIlford Sound boat ride and found it to be interesting. At one point they pulled up to a waterfall for some people to get drenched. Mind you this was in the winter. Queenstown is a resort town so there is a lot to do in the surrounding area along with many good restaurants and bars. Bungee jumping at its origin, the Nevis Swing and Shotover boat rides are some of the many things to do (which I didn't and now regret). There are also many wineries in the area. We did not go to Invercargill which I also regret not doing. My daughters friends from the area said that we would not enjoy Invercargill. We did go to Dunedin and Lamach Castle and the Albatross center. It is true that the roads are mostly 1 lane each way and travel times may be more than you expect. We hit some thick fog on our drive back from Dunedin since you drive through hills and valleys.
Beautiful country and we would like to go back again.

I know this reply is a bit late. Yes driving times are long ... and even longer when you are going through the sections where you want to stop every 10 min to take in the view!. We loved Dundedin and a day trip out to otago peninsula (hope that is the right name - it was a couple of years ago now). We loved the trip down to Milford sound. Dont get suckered into taking a coach day trip from Queenstown. It is a long drive and yes, there is not much at the sound itself other than the boat trip (which is awesome) but the drive is beautiful and worth stopping and getting out and walking around. We did it as an overnight to break up the drive. Queenstown is great. We are not into bungy jumping anymore (did it when we were younger in Zimbabwe) so the shotover jet was the only one of the "adrenaline" activities that we did but still loved it. If you have seen glaciers elsewhere - they are very underwhelming. The glaciers themselves arent as spectacular as elsewhere but if you enjoy walking,. there are beautiful walks out to them. Strangley one of our highlights was all the hot chocolate (we were there in winter) everywhere we stopped had the best hot chocolate!!! - there is a cafe on a corner driving from Queenstown towards Milford sound - probably couldn't locate it on a map if my life depended on it but the best hot chocolate in the universe!!

The Franz Josef glacier is an incredible hike and the views are spectacular!

Definitely take a look.

The thing that makes the NZ West Coast glaciers different is that they end going through rainforest. It is a wonderful experience to stand at the foot of a glacier surrounded by the sound of the forest birds!