Depends on what you like, and what time of year you’ll be there. You can do a driving tour of the South Island, but that’s a lot of driving. (I rented a rent-a-wreck, which was a really cheap option.) I’m an active person, and love to sight-see but get ansty when I’m in the car for too long. I’ve been to the South Island four times now, so I’ve gotten to see a lot. My favorite places were the Marborough Sound area (I took a boat trip to one of the islands to see birds), Abel Tasman park (took a kayaking trip), the West Coast (lush and dramatic), the Kaikoura peninsula (dry and dusty but beautiful tidepools and coastal scenery, plus you can go whale watching), and the Southern Lakes district. Lake Wakatipu took my breath away. The West Coast has some amazing scenery, and is very green and lush. You can do rainforest hikes near Haast. Near Hokitika or Greymouth (can’t remember) you can do a spectacular “glow worm” river rafting trip that takes you into caves, where glow-in-the-dark insect larvae glow on the ceilings like stars. There’s a basalt column formation that’s spectacular (Punakaiki Rocks), and you can hike right up the Franz Josef glacier, nestled amid a forest. Very odd to see a glacier below the tree line. I loved Christchurch, so spend some time there. The bars were super fun, and stay open till 6:30 am in the summer. Right near Christchurch is the Banks peninsula, which is surprisingly beautiful. I also loved Lyttelton, a port town near Christchurch, because it’s so quaint. That’s where our research ship loaded and unloaded supplies for our trip to Antarctica, so I’m fond of that town. Not much reason to go there otherwise, unless you want to take the tram to the top of the hill and do some sightseeing/hiking. It’s a flooded volcanic cauldera.