Africa - JNB<=>CPT Day Trip, what should pick?

I'll spend a day from JNB to CPT day trip return, since I had not been to Cape Town before, any recommendations or suggestions of Cape Town sightseeing places is a must?

By plane / by tour bus? If by plane (South African)

SA 317 JNB at 09:00, Jul 30 CPT at 11:10, Jul 30 342
SA 372 CPT at 19:50, Jul 30 JNB at 21:50, Jul 30 738

Will 8 hours of touring around town is enough? How far from CPT to the downtown area?

Tight, to say the least. Is there no chance to stay at least one overnight? ISTR that CPT to Cape Town is about half an hour by cab on a good day, perhaps double in the rush hour.

In 8 hours, you'll have to be very selective. Table Mountain is iconic BUT the cable car does not operate in strong winds, so this is difficult to plan. You might just have time to get to Robben Island if you book in advance, though this can also be weather-dependent. For me, these would be two musts which you would probably miss on a day trip.

You could hire a car and drive to Cape Point. There are also 4-hour tours from Cape Town. You could take part in a township tour. You could have a very decent lunch (there is an interesting thread on restaurants in the Africa forum). If you're into flowers and plants, you could visit the amazing Kirstenbosch gardens. You'd be unlikely to be able to do more than one of these. So much to see, so little time!

I can't help feeling that you'd be better off extending your visit if you can or coming back later. Good luck!

Well considering you arrive at 11:10 and need to be back at the airport by 21:00......I would seriously consider that you look at the possibility of getting a private vehicle and driver for the day - you should probably be looking at around R1,200 - R1,500 for a vehicle and private registered guide.
That way you have complete flexibility and someone knowledgeable who can take you around without wasting any time. in addition, being July, it is winter so what might be good on a sunny day could be just terrible on a windy rainy day.
My personal option would be have a driver at the airport and depart directly for Cape Point - you should get there in time for lunch if you wanted to, then return via the penguin colony on Boulder's Beach, do a wine tasting at Groot Constantia (South Africa's oldest wine estate) and then back into the city to enjoy dinner at the V&A Waterfront (touristy my a great setting for a once off) and if it is a good evening you may even be in time for a sunset cruise.
Then back to the airport.