Now, whilst I wouldn't fly to JNB just to see the city - there are some interesting things around - especially if you like to see urban renewal and new things happening - bit like finding new things opening in gritty areas of Dalston/Shoreditch in London etc. I spent most of last year in JNB and found some good things to do on the weekends:
The best tour to do of Soweto is a bicycle tour. The Soweto Backpackers B&B on Pooe Street, Orlando West, Soweto does them - search on the web for Soweto Bicycle tours. You can drive there (note - get the right Pooe Street in your sat nav) or get them to collect you from your hotel. Orlando West is safe enough to drive to during the day - take a morning tour.
On Saturday mornings the Neighbourgoods Market is on. This is the same Market that run at the Old Biscuit Mill in Woodstock, Cape Town. The market is a foodie one - and it is in a student area called Braamfontein, just before the Nelson Mandela Bridge. The address is 73 Juta Street. There are several new art galleries and cafes that have opened in this area - and there is parking on the street, and a multi-story - follow the chap with the market parking sign.
This area feels a bit like Shoreditch in London - it used to be dangerous, but is now an key area of renewal. The Kitchener Bar here is old and fun in the evenings. There are several new hotels opening here, and the Lamunu Hotel is cheap, stylish and good on Melle Street. An American woman opened a restaurant called Narida Trogans on De Korte Street a few yards away - nice but only open weekdays and nights.
If you have driven down - then you can visit the Johannesburg Art Gallery 5 minutes away at Joubert Park. There is secure parking - and it has some interesting smaller pieces of famous artists. However the most interesting thing is walking around the secure car park and see the decline of Hillbrow from a safe place. Do not walk here from the market - only drive.
On the weekends, there is another market called Arts on Main in the Maboneng District - it is actually on 264 Fox Street. Search online for this. It is quite good - Sunday is best as there is a food market (marketonmain.co.za)
also. There is a rather oddly office shaped hotel beside it called the 12 decades hotel. There is a rooftop bar that holds a party most Sunday days called Party on the Roof. It is very good, dance music playing as you look out onto the urban expanse of JNB - a place where blacks and whites mix with quite a big black gay crowd dressed to the nines. Much more interesting than the sterile bars of Sandton!
Melville and Parkhurst (around 4th Avenue) are also worth a look - check the guidebooks. Also the 44 Stanley Market - a collection of shops in old courtyard near Melville is worth a look.
A website that I found useful is www.todoinjoburg.co.za
- it is a blog of various photographers suggesting to do things in the city centre. Some great suggestions - there is one about an Ethiopean community tour round JNB City Centre if you can find the details.
Anyhow, I hope this is useful - when I first visited, I was told not to go outside of Sandton. Once I drove to Soweto and round the centre after trawling the web, I realised there were interesting things to see. Sandton is a ghastly anonymous area - all that is there is a huge shopping mall and some overpriced restaurants and bars - you might as well be in the USA or Europe.
PM me if you want more City Centre info!