There's not a huge amount to do in JNB. Sandton City has very good shopping and the IC there is pretty good. You could also go on a gold mine tour but I wouldn't prioritise Jo'burg.
CPT is more interesting (Table Mountain, Robben Island, Cape peninsula, winelands) but it will very much be winter there with weather that is worse than the rest of SA.
Head up to Kruger/Sabi Sands to go on safari. July is a good time of year to go because it's the dry season and there won't be so much foliage. Malaria is also less of a problem at that time of year.
If you want to dive, there are plenty of coastal reserves. Sodwana is vey popular but it's a bit of a faff to get there. The closer you get to Mozambique, the warmer the water is. Around the Cape, it's pretty chilly.
Somewhere around Durban could be a good base for diving/shark-related excursions.