I have often gone off and done things on my own, instead of the cruise ship excursions.
It's not just about the money - it's also nice to be able to stay as long as I want, and do what I feel like, without being on someone else's clock. That's a vacation to me - doing what I feel like, when I feel like
That said, lots of times if the trip is complex, it may be easier just to take the ships excursion