Pretty good avice from Roger, I think.

Another variation is to fly JNB/PLZ then rent a car and drive to Cape Town. Addo is good, as Roger says, for an upmarket experience look at Shamwari

Knysna would be a good place to stay 'en route' too.

Namibia would be too far to drive comfortably (and the drive is not interesting at all). But if you fly up, you could of course do Etosha (but the same applies as Roger mentioned for Kruger: vegetation (although not as much as Kruger) and malaria). Etosha is in my humble view nicer than Kruger (fewer tourists) although I have not been in either for many years (I am a sort of 'once you have seen one leopard you have seen them all' kind of person - this is sacrilege for someone living in South Africa but there you go...)