Just did a walking tour of the CBD which was really interesting. It's run by and starts at Curiosity Backpackers in Maboneng. You can easily get there by the Gautrain to Park and then a cheap Uber across town.
http://www.mainstreetwalks.co.za/
I highly suggest a Soweto tour using the Tuk-Tuk through these guys. You can also reach the meeting point safely by taking the Gautrain to Park and then Uber or if you're more adventurous the Rea Vaya to Orlando Stadium and they'll pick you up. They generally do a tour at 1000 and 1400ish.
http://www.sowetobicycletours.com/tuk-tuk-tour.html
If you're interested in a driver I can recommend a really good guy with reasonable rates.
Cheers
Howie