Keeping in mind the duration of yoyr trip, I would suggest you visit the following places - Knysna, Wilderness, Storms River, Plettenberg Bay. If you visit Knysna, try to do the day trip to [broken link removed]. We thoroughly enjoyed it, and so has everyone we know who did it.
If you're lucky enough at Plett, you can sit on the rocks at the Beacon Isle end of the beach and watch the dolphins go by.
Do a search and you'll find quite a few discussions about where to go and where to stay on the Garden route.