I think it all depends on what area of the south island you are going to base yourself in.
I prefer the Southland/Otago areas, make sure you do check out Christchurch and Dunedin on the way through though - the original architecture is awesome, especially in the Dunedin CBD/The Octagon.
Take a drive down from Dunedin through Milton and turn off to go across the country via Roxburgh & Alexandra. If you like fresh fruit this road is absolutely littered with orchards selling all manner of fruit from roadside stores, fresh from the trees/bushes. There are many pick-you-own places too, where you get to traipse through the orchards and pick your own fruit.
Also, try a Jimmy's Pie - their factory & store is at the entrance to Roxburgh. They are divine.
It's old goldmining country, so there is like 7 pubs along that stretch of road, which I find quite amusing.
Make sure you check out the little cathedral at Lake Tekapo - it is beautiful.
You could go Queenstown which, while it may be extreme sport central, is also a nice place to just hang. Take the gondalas up the hill, the view is glorious. Also the TSS Earnslaw is a magnificent paddle steamer on Lake Wakatipo which does tours of the lake and visits farms and whatnot. The lake itself is beautiful, but bloody cold!
A lot of the LOTR films were shot around that part of the country too and I'm pretty sure there are tours.
You could head over to Te Anau and have a look at the scenery, there are a few glaciers dotted about the place too. Plus the Milford Sound along the West coast.
That's just the south of the south.