Hi Penny,
I did this tour last Sept and a couple of the group went to Robben Island, some had booked before going and the tour manager helped a couple of others who wanted to go to organise it once in Cape Town - but I wouldn't guarantee that you would be able to get tickets for the day you want to go. However, you would be missing the winelands tour through wonderful scenery - we went to three vineyards, including the world famous Nederburg and had tastings in each, a brilliant and fun day! The other full day in Cape Town we went on a Cape tour - stunning coastal scenery, the penguin colony at Simon's Town and more, another fantastic day.
Best wishes,
Bob