Visiting Johannesburg

That's an odd one Pouya ... you wouldn't really bother with Johannesburg unless you had existing business there (most national head offices are located there). It's true that there are a few 'touristy' game farms set up around Johannesburg to give visiting foreigners there on business a quick taste of the African experience, but your better bets are much further away (e.g. the famous Kruger National Park to the north, or the Addo Elephant Park in the Eastern Cape).

As for 'fun things to do in Johannesburg', I think people from Johannesburg are still trying to work that out for themselves Entertainment is basically visiting the nearest mall or bar, and for that you can just as well stay in Cape Town.

If you do find yourself in Johannesburg, it all depends on whether you're interested in South Africa's cultural history (there are some museums if that's your sort of thing), or for the true 'touristy' experience you have to visit Montecasino (a casino that also has a quaint indoor shopping and dining district). To answer your question, I wouldn't allocate too many days to it. Also, for safety reasons, don't think about doing too much walking.