In December, Cape Town is generally full of holidaymakers, which makes it quite interesting (but Johannesburg is dead). If you must come in September then you are taking a bit of a chance with the weather but Cape Town might be warm even hot and still be hosting overseas holidaymakers.
There are a few game reserves located near Johannesburg such as the Pielansberg Park which is only about a 2 hour drive from Johannesburg. I do recommend staying in a good lodge with organised game drives so you get to see all the wild animals! Take at least 2 days for this little trip.
Sun City, the casino resort is also located in this area.
In the Johannesburg / Pretoria area there are some historic monuments, some museums, a decommissioned gold mine, a couple of fancy shopping centres and casinos. Stay in decent hotels with good transportation facilities (taxi’s, shuttle busses) near commercial areas such as Sandton or Rosebank in Johannesburg.
In Cape Town I would recommend staying in the Sea Point/Clifton area. The beaches are magnificent but the water is freezing cold.
In Cape Town you could do the Stellenbosch wine route and go up Table mountain in a cable car (or hike it) if you have the energy. The Cape Town scenery is beautiful.
Please make sure you are alert of your surrounds. Don’t go walking around at night!