I agree with the above, with the exception of worrying that private tours won’t get you back on time. If you hire a private or semi-private guide you have the benefit of reviews and other cruisers’ experiences to ensure you get someone reputable. I’d actually be more worried about someone I picked from a group of guys on the pier. Any company with any presence on TripAdvisor will not want a “they made us miss the boat” review.
As for what to do, there are so many options. We like to mix history with doing things like cooking classes, food tours, beach time, etc.