I support the above comments about Jo'burg -- the city has not much to offer and is pretty dangerous (I have worked/lived/gone to school in some of the worst sections of US cities, and I was nervous walking downtown).
Having said that, if you can swing a few days, definitely go to Kruger for an animals -- a great experience. Since you are already going to CPT, I might recommend a quick flight to Victoria Falls -- cheap, quick flights. If you go, land in Zambia, NOT Zimbabwawe -- the latter is extremely dangerous and full of hassles.
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Da DOK