Capetown ("Kaapstad" in Afrikaans) is one of the most beautiful major cities in the world. It is the meeting point for two oceans with mountains and a natural harbor. Thanks to its nature many migrants moved there and movies were recorded there. Activities may include surfing, fishing, shark watching (!) or just walking and enjoying. Note that the area also produces good wines...
Johannesburg is the largest city and economic heart of South-Africa. People are starting to make it a "friendlier" city, away from the bad name of robbers and thieves. You still need to be careful, but a city tour to see the old and new Joburg is certainly worthwhile. Buying diamants? De Beers has its HQ here. Politics? Visit So-We-To (South-West Township) where Nelson Mandela lived.
The Kruger National Park is one of the largest game reserves in the world. The big five and many other animals have their home here. The location north-east of Johannesburg makes it an easy destination.
Blyde River Canyon (Dutch: "happy river canyon") is one of the largest and certainly greenest canyons on Earth.
Stellenbosch is a wine area close to Capetown. It is named after its Dutch founder Simon van der Stel.
Durban is the 3rd largest city and the busiest port in South-Africa. Durban is a ethnically diverse city, with large Zulu, White, and Indian/Asian populations.
Port Elizabeth is one of the largest cities in South-Africa. Portuguese, Dutch and English history left its traces.
Hermanus is a city on the South-Coast where you may watch Whales from the coast, without the need for a boat, when the season is right.