Your itinerary seems do-able.

For Cape Town, cable car (or hike) to Table Mountain, and also see who does parasailing since you guys seem to be into that.

Vineyards, (if you drink) - there are plenty.
Golf??
Shark diving.
Have a meal at Kalkies at Kalk Bay -- an experience.

Remember that it will be high season, expect queues and not so good pricing, but the weather should behave.

For JNB, you could go the Lion park (if you have not seen lions yet), Maropeng (where they have just announced new fossil finds).

Hector Pietersen Memorial, Apartheid Museum.

Enjoy.