I have spent almosg two weeks in Johanessburg. Quite a lot. You dont need more than three days. I was supposed to head to Mozambique but the visa fell through and I had to extend my stay in Joburg. I spent most of my time in Maboneng and a couple of days in Braamfontein.
There are some must to dos in Joburg and everything is about the history of this city and the history made in this city.
South West Township or Soweto is a must visit place. I did a day tour which combined Soweto with Hector Pieterson Musuem, Constitution Hill and the Apartheid Museum. All four locations are critical to understandind the dark history of South Africa and the impact of Apartheid. Soweto is iconic as both Nelson Mandela and Desmond Tutu stayed here at some point. It also showcases different elements of South African history including segregation of communities, povery, positive changes and modernism. The Constitution Hill is about the freedom charter and showcases a step forward in its history. Hector Pieterson was a young black student who was killed in the student riots of 1976 where students, rightly, were protesting against the policies of the apartheid regime. The Apartheid Musuem takes the explanation a few stepa forward. I would recommend atleast half a day for this musuem.
In addition, I would highly recommend the Origin Musuem (History of Man through fossils, tools and culture including the sad history of the San people and about Sara Baartman: Google her and learn more about the fucked up nature of then racisim) and the Wits Art musuem ( exhibitions change: I saw a photographic exhibition of flooding and its impact on global communities).
Joburg has so much history including the impact on Mahatma Gandhi and the Gold Rush. You can learn more in Gandhi Square and the fact that Anglo American (They also own De Beer) is based here. I wish Joburg Tourism pushes its history status a lot more. The city is generally unsafe but with growth in tourism, things will improve (And it has in the last decade).
Two amazing highlights: Neigbourgoods market ( Saturday only) in Braamfontein and Mabonend (Fox street) with its cafes, restaraunts and bars.
In Maboneng, I would recommend Chalkboard, Eat your Heart Out, Origin Coffee, Pata Pata and Little Addis Cafe. I have been here. There are other places but havent eaten there.
In Braamfontein, I was low on cash so didnt spend a lot eating out but did have dinner at the Immigrant and the food was very good.