Agree with the above.

If you are the type that enjoys lurking around airports and lounges then OR Tambo is quite OK. The City Lodge is excellent and attached to the airport (the Protea is a transit drive away). Inexpensive for a day room and easy to work there (though I believe you need to pay for the wifi).

Personally I would recommend taking the Gautrain into town. Sandton and Rosebank are both pleasant for a wander and something to eat, or even across to Pretoria where in a day you could easily do Union Buildings and Voortrekker Monument.

Alternatively take a taxi from the airport and visit the Apartheid Museum. Less than £10 each way and will take up 3 hours comfortably.