It is possible to visit the Sterkfontein caves but you have to find your own way to get there. They do tours of the caves for a nominal fee (tips are optional) but don't expect to see many fossils. The archeolocial digs were all closed off.

The caves are impressive and it is a good day out, considering the many unique private restaurants in the area. That way you could drive around the Highveld countryside enjoying some time away from the city bustle of Johannesburg.