The stretch between CapeTown and PE (as the locals call it) includes the beautiful Garden Route. All the places MJW mentioned can be found along the Garden Route.
I have a personal journal piece about whale watching in Plettenberg Bay here:
http://journals.worldnomads.com/philsylvester/story/58691/South-Africa/Whale-watching-in-South-Africa over 6 years ago
The Garden Route is spectacular, and I would recommend you take your time to explore the areas that you will visit. There are great little antique shops, farm stalls, beautiful scenery as well as the usual tourist activities. It really depends what experience you want out of your time and what interests you most?
You can read more about the Garden Route and what activities are available here:
One of my personal favorite experiences was driving over the Montagu Pass - the scenery is truly breathtaking, great sense of adventure and history. http://en.wikipedia.org/wiki/Montagu_Pass.
One of my best places to stay and eat is: Emily Moon. http://www.emilymoon.co.za/emilys/index.html over 6 years ago