Hi,
In Cape Town the open top bus tour of the city is good for getting the lay of the land.
If the Table Mountain cable car is open on your first day go for it as the mountain can cloud over easily and remain closed for several days in a row.
Other places worth seeing in and around Cape town include - Robben Island, District six museum, driving down to the Cape including Chapman's Peak and the penguins on Boulders beach at Simonstown.
Franschhoek is a nice town to visit in the winelands...it's a lot smaller than Stellenbosch and has countless wineries in the area, lots of accommodation and some of the country's best restaurants.
Boschendal is very nice for tastings as is Grand Provence.
If you are there in time for lunch the lunch in la Petite Ferme above the town is a very pleasant experience.
Several of the wineries also offer picnics in the vineyards.
http://www.lapetiteferme.co.za/restaurant.htm
http://www.grandeprovence.co.za/
http://www.boschendal.com/
In the Oudtshoorn area, if you'd like to put your car through its paces, take a drive up the Swartberg pass. It's a great drive (on a sometimes steep, narrow and unpaved mountain road) through some quite remote and rugged landscape, and you can come back via the town of Prince Albert and another mountain gorge. Takes a couple of hours round trip. Jemima's is a lovely restaurant in town...we had lunch and dinner there!
I see that you are staying in Oudtshoorn and Knysna for one night each. Be aware that you'll more than likely arrive in Oudtshoorn in the afternoon if you are driving from the winelands, and again, if you wish to do anything in that area the following morning it will be the afternoon by the time you get to Knysna.
I guess I'm just saying that you should enjoy the drives and the views along the way as you may not have a massive amount of time in either town to 'smell the roses' unless you drive like crazy...which would defeat the purpose of driving Route 62 to Oudtshoorn in the first place. By the way, "Clarke of the Karoo" roadside cafe in Barrydale is a nice stop for lunch along the way from the winelands!
Along the garden route we stayed in Kynsna for a couple of nights, and to be honest found it to be a bit of a let down given the amount of hype it gets. Still, the Waterfront area is nice in the evening and there's lots to do in the area. Nearby Buffels bay and Brenton on Sea are nice for a drive and a swim or surf, and The Heads area of town are nice for a look around too particularly if you like ogling luxurious houses. Plettenberg bay looked nice - we passed through a couple of times- but didn't see the beach.
There are endless activities within about 30 minutes or so of Plett so depending on your interests you'll have no shortage of things to do, from jumping off the highest bridge in Africa (with bungee cord attached) to surfing, canoeing, walking etc.
You'll have a great time, enjoy.