Depending on the time of year (sorry I cant remember but certainly the peak-possibly Christmas-ish and local holidays) there can a major wait for the Robben trip – sometimes over a week. You can book in advance. Otherwise in half a day – a wine tasting, the Table mountain trip (be a bit careful) or go and see some penguins!
Edit – I’m no expert – Ive only been a few times but come to think of it in a half day – a trip to Franschoek / Stellbosch (sp?) should be doable – certainly in a hire car. If driving yourself try not to return after dark for the first time (avoiding certain bits etc).
Generally youll have a great time – its not nearly as threating as you might expect and if youre sensible you’ll fall in love with the place.