All very good advice from worldiswide above!
Just to stress: Do the Table Mountain trip at the first possible opportunity. When it is too windy or cloudy, the cable car does not operate and that can - in extreme cases - be for days at a time!
The remark by the shark boat captain (maverick17's post) about people getting left behind on top is urban legend. If the weather deteriorates, they sound a siren to call you back to the upper cableway station. So no stress about that. And the alternative, walk down Platteklip Gorge, is relatively steep and not easy on the knees, but very doable. Wearing relatively sturdy shoes is a good idea anyway if you want to wander around on top a bit, away from the cableway and the restaurant. Its perfectly safe there. From below, the top - a bit away from the station - looks flatter than it actually is. Worth walking around a bit and spotting the rock rabbits ("Dassies").
Book the Robben Island ferry in advance. The ferry is notoriously unreliable and is often replaced by a smaller craft, in which case they won't sell any tickets on site.