There's enough to do in Johannesburg especially if you're only here for a few days. The South African Museum of Military History is a very place and there are other attractions such as the Lion Park.
Insider Tip:Always check with the hostel you're staying at beforehand about what you should be paying the taxi drivers to your various destinations. Also check that the taxi driver - before you get in the cab - that they know how to get to where you want to go. Johannesburg is a big place and anyone can become a taxi driver and often the taxi drivers have no clue about different destinations, and, they don't seem to know how to operate a GPS or read a map.