Ceasar's: Close to airport
If I understand correct you will just be in JHB for the evening/night. You will have to take a taxi into JHB. Remember the airport is about 20-25km away from JHB's CBD. That will cost a bit. As the previous poster said there is not much to do in such a short time. There are theatres, cinemas, clubs etc. If you are going to stay at one of the airport hotels Ceasar's might be your best option. It's a casino but there are also shops, restaurants, bars with live entertainment ( you can check that on their website), a theatre etc. It's 5 min from the airport. The Southern Sun shuttle might take you there, not sure but a taxi there won't be too expensive. Just use a metered taxi and not a mini bus. It would be perfectly safe to walk around alone there. There are lots of people. Don't walk around alone outside when it's dark.Hope it helps.
