I wouldn't worry about waitlisted excursions, Marc, they almost always clear -- if not before the cruise, then during the cruise.
As for tours in Capetown, remember we don't even dock until 4:00 p.m., so the tour options after that would probably be pretty limited. Who wants to start a 4 or 8 hour tour at 4:30 in the afternoon? At least, not me. In the past we docked right at the V&A Waterfront which has gazillions of shops and restaurants--a perfect place to spend an evening--much better than a tour bus.