If you would like to get the most out of a day in Cape Town I would advise to rent a car and just drive south, passing places like Muizenberg, Fish Hoek and Simons Town along the way down to the Cape of Good Hope.
With a group of fellow students we picked up our cars early and headed straight to Hout Bay, where you get an excellent view of Cape Town (see picture), and the roads here are in between the hillside and the water, which make this first part a beautiful ride.
From Hout Bay we decided to drive through Muizenberg and down to Fish Hoek which was an excellent place to have lunch at one of the many restaurants close to the beach. From there it was just a short ride over to Simons Town, which is the city nearest the famous Boulders Beach, where they have a Penguin Reserve. The penguins were interesting enough to watch, but after a while their smell catches a bit up with you and it is time to move on to the highlight of the day: Cape Point Nature Reserve. The first time I took this drive I was not aware of the nature reserve closing down at 5.30, so I had to return to Cape Town without getting to the goal of my journey, but having it all planned out we made it within the opening hours this time. Once we had entered the nature reserve there were wild ostriches, bamboos (that jumped on the roof of our car) and springboks (the national animal of South Africa) along the whole road to Cape Point. Having seen the viepoint from the top and taken a picture with the sign we returned satisfied back to Cape Town after the perfect day trip.