I found Victoria falls worth it, and adding Zambia/Zimbabwe makes the trip a bit more exotic. South Africa is very developed compared to much of Africa, good quality roads, airports etc everywhere, so it's interesting to see something a bit different.

We stayed right on the falls on the Livingstone side and walked across the famous bridge into Zimbabwe. Also did a river safari seeing crocs up close. The combined Kaza visa for Zam/Zim also let's you do a day trip into Botswana through the Kasane ferry crossing so you can do a day safari into the Chobe National Park which is offered by a few companies. You only need two days though - flights to either Livingstone, Zam or Vic Falls, Zim on BA or SAA leave JNB in the morning and return early afternoon. There's also a less than daily flight from Livingstone to Kruger.