To travel to South Africa you have three major choices. The most direct one is driving to Cape Town South Africa down the infamous Namibia to Cape Town route. You are looking at around 1450 km to Cape Town crossing the border at Noordoewer across the mighty Orange River. Taking in the sites in Namibia you make your way to Vioolsdrif across the Namibian border, the South African border post. Now you can relax all the way down to Cape Town.
Another route to cross the border into South Africa is to travel through the Kalahari Desert entering and exiting Botswana on your way down South to Johannesburg. Most rental cars can be returned in Johannesburg city centre or at the Oliver Tambo airport. Crossing the border to South Africa is easy just make sure you have the necessary paperwork that allows your rental car into South Africa.
A great way to cross the border into South Africa is when driving from Windhoek to Mariental and then onto Tweeriver and the entering the Kgalagadi Transfrontier Park crossing the border at Mata Mata which is also a camp you can stay in for the night. Now enjoy this cross border Park experience that shares a eastern border with Botswana so the Park is made up of a larger section in Botswana and a smaller section south Africa where the majority of the animal are concentrated. You can then leave this cross border Park head South and exit the Park border at Two Rivers South Africa and continue to Upington. Here you can then turn left to travel via Olifantshoek to Johannesburg or turn right and visit the mighty Augrabies Falls in the Augrabies Falls national Park and the onto Cape Town making this a very adventures trip with lots of highlights to see.