Interesting and safe things to do in Johannesburg on less-than-whole-days

I’ve been summoned, somewhat surprisingly, to join a consulting project in Johannesburg, South Africa, where I will be flying tonight. I’ve never been to South Africa before (nor, for that matter, anywhere else on the African continent), but I’d like to get at least some touristic value out of the stay.

The work I’m doing will be consulting, meaning that I’ll be busy over the day and will have spare time in the evenings, and one whole weekend, before leaving. Now I hear Johannesburg is not really the safest place on this planet, particularly in the dark. Do people know of sights or activities there that might be of interest to a traveller with an interest in history, but always only a couple of hours of spare time at hand in the evenings? I bought a guidebook, but these are, obviously, aimed towards the full-time tourists.

Any ideas or advice would be greatly appreciated!