After arriving in Johannesburg you could drive to Sun City, The Palace of The Lost City there is quite cool. Pilansberg NP is not far away, so you could do a day safari there (good chance to see rhinos). Or/and before Kruger, fly from Joburg to Nelspruit and drive the Panorama Route, very nice scenery with God's Window, Three Rondavels etc. to Phalaborwa and then enter Kruger NP. Or you could visit the east part of SA and enjoy St Lucia / Isimangaliso Wetland Park before flying from Durban to George / Port Elisabeth. There are soo many nice options!!