The South Island of New Zealand is characterized by grand open landscapes. Divided by a backbone of mountain aptly called the Southern Alps, the South Island is known for spectacular mountains and fiords, large beech forests, golden sand beaches and broad plains. Generally cooler in climate than the North Island but don't forget sunhats and t-shirts - temperatures are routinely in the 30°C's in summer. In winter the sea buffers the temperatures which rarely drop much below 0°C except in mountainous regions.
The South Island of New Zealand is the larger of the two main islands though it has fewer people and is sometimes referred to as the 'Mainland' - especially by South Islanders. Geographically the South Island is dominated by the Southern Alps. Dividing the island, the alps affect climate and flora. Most of the South Island's national parks are strung out along the main divide.
Generally, the West Coast is wetter and cooler than the east, and the north of the island is warmer than the south.
Christchurch, Dunedin, Invercargill and Nelson are the main settlements, although the main attractions are rarely in the cities. All four cities are very different. Christchurch is the largest and has a certain English feel to it though it is definitely a New World city. Dunedin was settled by Scottish Presbyterians and is very proud of those roots. It also feels older than other cities in NZ because it was built by gold rush money in the late 19th century but has since been surpassed by bigger and brasher cities to the north. Nelson is still very young by European standards (although it was the second founded city in New Zealand) but has a very South Pacific feel with palm trees and a huge and beautiful white sand beach.
It should be noted that beautiful beaches are a dime a dozen in NZ, and some of the best do not average one visitor per day.
The Interislander and the Bluebridge  ferry companies run from Wellington to Picton through the Marlborough Sounds and across Cook Strait. The ferries take cars, buses and trains. The scenery on a good day is spectacular. The ferries are substantial ships designed for the sometimes rough conditions and the journey takes between 3 and 3 1/2 hours.
Two standout train routes are on the South Island. The Picton - Christchurch Tranzcoastal begins traveling through the Marlborough wine region before hugging the Kaikoura Coast and crossing the Canterbury Plains. The Christchurch - Greymouth Tranzalpine crosses the Southern Alps at Arthurs Pass. Rated as one of the most scenic train journeys in the world.
Buses are a cheap way to get around to the main centers of the South Island. There are a range of types of services, from a luxury coach service to minivan shuttles. Shuttles which service a local area can be found in the regions and towns which they service.
Roads in the South Island vary in quality and traffic, but as long as they are treated with respect they serve you well. Rental cars are available in most sizable towns. The best range (and hence lowest prices) are in Picton (off the interislander ferry) and Christchurch.
Internet based rideshare systems are growing as fuel becomes more expensive. Jayride  is a good ridesharing site designed for specifically for carpooling in New Zealand. See the New Zealand page for more options.
The South Island has become the home of Adventure Tourism. That is, ordinary people being encouraged to do crazy things; such as jumping off a bridge with a rubber band tied to their ankles, riding in a jet boat or rubber raft.
Things to do include: