I would say two to three nights is all you need on Safari. Apart from the Kruger Park, there is also Pilanesberg National Park which is right next to Sun City and has the “Big Five” on display. Its only two hours from Johannesburg and its also a maleria free area which is a bonus so no anti maleria tabs need to be taken.
Cape Town is a must, with Table Mountain, Robbin Island and all the wine farms in the area. You can rent a car and take a drive down the Garden Route to Knysna, staying in B&B’s along the way. Really breathtaking scenery and lots to see and do.