We've always booked our own excursions on cruises and haven't ever had a problem. You have to do a lot of research on your own though. It is usually MUCH cheaper to plan your own. I just "google" the "port" and come up with lots of info. With 10 people, you could possiblly book a van in most locations to take you all where you want to tour.
We're going on our 1st Med. trip this spring also.
I know that Dubrovnik is a walled city and is "supposed" to be pretty easy to walk on your own. Kayakolon is the ONE port where they say you should consider booking the ship's tour, b/c otherwise there's not many alternative transport options there to take you to Olympia.
I also get tons of books from the library to study each port ahead of time. Tripadvisor.com is also another great website with forums and trip advice.