Here's advice I was given about a brief overnight, certainly a different scenario and yet about the same number of elapsed hours.

http://www.flyertalk.com/forum/afric...citylodge.html

CityLodge mentioned above was recommend to me in that thread. There is also a transit Protea hotel not discussed in that thread. And several encouraged me to go into Jo-burg which is quite easy to do with the Gautrain. You might find you want to go get a meal as a means of stretching your legs and breathing outside air.