Hi Peter, I'm sure you've finalised plans by now but just to add - if you're staying in or near Cape Town I'd recommend a visit to Kommetjie if you have any spare time. There's probably nothing there that you won't see elsewhere but photo opportunities are good.
I'd thoroughly recommend an overnight stay in the West Coast National Park even if you've paid for accommodation around Cape Town. You'll practically have the place to yourself and the birds are very approachable around the chalets. Beware though if you go, low tide is just as important as high tide. It doesn't always go out far enough and many an hour have been waisted waiting for the water level to drop!
As mentioned above, Kirstenbosch is an absolute must along with Harold Porter Botanical Gardens, the penguin colony - I could go on.
Good luck with the pelagic, it was the highlight of my trip in March last year following the disappointment in 2013 when the weather was against us all week. I'm sure you'll have a great trip.