Ha ha. One day or two? Most people around here say 4 days minimum so I would definitely go with 2 days. If you're worried about getting lost in the city, book a room in a hostel. A room give you space in which to be boring while having the better hostel infrastructure for meeting up with other travellers. For Stellies or Franschhoek you won't have that problem as you're going on tours anyway.
With two days in Cape Town, I would do the hop-on-hop-off tour bus with a long stop at Table Mountain to go to the top. And for the second day I would do either a peninsula tour or a Robben Island trip.
