You should definitely visit Cape Town. Domestic South Africa flights are very inexpensive, so I'd recommend making the journey by air.

Victoria Falls would also be a worthy side trip. You could fly into LVI or VFA, but I'd recommend staying on the Zambian side. If you go to Livingstone, the day trip to Chobe National Park in Botswana is also good. Of course, if you do go to Zambia, you will need to get a yellow fever shot to come back to South Africa.