When you drive down towards south, all of the small beaches are very nice. Camps Bay is beautiful and has plenty of beautiful people and restaurants - but is maybe a bit too touristic. Llandudno is very beatiful, but remember there are no shops or cafes. Hout Bay is a charming little town with a fish harbor and a long beach. The water by the Atlantic is rather cold and waves are big.
On False Bay side the water is warmer and calmer and the coast is full of nice small towns. Boulder Beach with penguins is fun, they also have a nice restaurant. Simon's Town has all kinds of little stores. Kalk Bay is my favorite, with art galleries, antiques stores, cafeterias, fish harbor and Harbor House restaurant.
Still on the other side of False Bay is Strand, the best beach around, also very suitable for kids. Here you can learn surfing, too. A bit further is Gordon's Bay and small and quite Bikini Beach, good for swimming and eating lunch at the yacht club.