Book as many days as you can and don't be lazy! Do some research
I think I could spend a whole year in this city and still wouldn't have the sensation that I did everything that I wanted to do.
After some 20 intense days there, there's still a whole list of things that I reserved for my next visit to it - and I didn't even do the clichê things, such as shark diving or bungee jumping.. (maybe I can do that on a 4th visit).
More than beaches (cold and boring, compared to the ones we have in Brazil) and beautiful mountains, CT's got great bars, markets...
Insider Tip:Have a burger at the Yard/Dog's Bollocks/Bitches Tits, a delicious "hipster" burger house on Roodehek St. which makes their own (and delicious) beer. Watch a rugby match (or even better, a Man-U match, and discuss football with the Leeds-fan owner) at Perseverance Tavern on Buiterkant St. Don't miss the already famous Old Biscuit Mill. Take a train to Kalk Bay and spend an afternoon getting drunk and swimming at Brass Bell. Wacth a concert at Kirstenbosch on a sunday (taxi $$$ :( )