Johannesburg is the most well known city in Gauteng and is the main airport hub for international flights and connections to other regions. The airport is located quite a long drive from the city centre out on the northern suburbs. There are several good standard hotels and lodges near the airport if you find yourself overnighting here. If you have a bit more time, take the chance for some great shopping, explore the markets or visit Soweto.
Famous or notorious, Soweto is a former township from the times of apartheid and is about 15km from Johannesburg centre. It started life as a temporary place for mine workers to live but stayed put and in 1923 was declared a black ghetto. A tour to see this huge town is a real eye-opener.
Pretoria, to the north of Johannesburg is actually the capital and administrative centre of South Africa and has a much more colonial, laid back feel to it. In the spring time, the Jacaranda trees lining the streets with their mauve carpets of petals is an incredibly attractive sight. There are museums and galleries and it's a pleasant place to spend a day.
Other things to see out and about round the province are the Botanical Gardens, Johannesburg Zoo, Gold Reef City or Randburg Waterfront.