Cangst,
Your trip sounds wonderful. We just got back (3 hours ago) from a month in Cape Town and driving through the Garden Route. We had 2 nights at Kwandwe and it is an amazing place. The people are so freindly and it was the highlight of our trip. Plet has beautiful beaches and good food, but Dec is high season so it will probably be pretty crowded. I wanted to stay at Phantom Forrest but we did not have the time. It sounded really interesting though. We did not make it to Knasa but everyone kept telling us to go. The wine country is so much fun. If you are the least bit serious about wine, I recommend getting the John Platters guide. It list all the vinyards and ranks the each wine that is produced (I may be able to send you my copy). Franschoek is a nice little town. We stayed at Le Petit Ferme. Great place. Spend a night in Stellenbosch. It is kind of a University town, but there is good food and it's nice to walk around. We also spent a night in Paarl at a great place called Roggeland Country House. It's quite well know for its food and the accomidations were lovely (there is also the Grand Roche but they close during August). Unfortunately, you will miss the whales in Hermanas . Driving from Knysna the Franschhoek will probably take 6 hours being it is high season (while the roads are nicely paved they are often one lane). As for the extra two days, I would not book anything now but wait until I got there, there is so much to do you'll easily find a way to fill the days. Hope that helped,
Take care
Chris