Common sense is the best guide.
In South Africa a pump attendant will be on hand to fill up your car at petrol stations. He or she will also clean your windscreen and top up the tyres if asked (2.2 bar all round). Generally petrol (unleaded in the Mx5) is paid in cash, a till receipt is given on request, and the pump attendant should be tipped a couple of rand for their assistance.
In the city there will usually be a car guard on parking duty wearing an official bib. You will be issued with a parking ticket, or be expected to tip 2-5 rand when leaving again.
South African road users often drive fast and sometimes unpredictably. Beware especially of the taxis (the white minivans) who are notorious. On one lane roads there is often the space to pull across the yellow line to the left to let others overtake. Slower vehicles will often pull over to give way. It is customary to show thanks with a flash of hazard lights in such cases, though with the top down, just wave!
Speed limits are signalled and cameras are in operation. So watch out for them. Finally, it would be extremely unwise to explore any of the townships without an experienced guide. So keep to charted territory, stay on the road, let the wind blow your worries away, let your senses revel, and you will enjoy the exhilirating best of the Cape Town Drives.