I have not been to these places personally, I can only tell you what we've decided based on LOTS of research and various forum contacts.
Over and over I hear these two things:
Monaco is not that great. Go to Eze or Nice, either on your own or a ship's excursion. Apparently bus and train to these towns are cheap and easy. If you want to stay in Monaco, DIY using the bus system.
Of all the possibilities in the Almalfi/Capri area, Sorrento consistently comes up as everyone's least-favorite. So we have decided to skip it. We are doing a back-2-back cruise, so will actually have two opportunities to visit the ports. One day we're doing Capri on our own. The next time we hit Naples we're doing a private tour going to Pompeii, Positano, and Ravello.
There are a number of private tour companies. After research and contacts, we narrowed it down to three for our Pompeii/Almalfi coast tour. All have excellent reputations.
Joe Banana, €450 for 8 people
APTours, €450 for 8 people
Rome in Limo, €600 for 8 people
Hope that helps!