Welcome to cruisereviews.com. I gather that you'll be onboard Nieuw Amsterdam. There are a few variations of the Magic of the Mediterranean cruise. Do you embark and disembark in Venice? Or, is either embarkation or disembarkation in Barcelona? No matter which one, it's a super itinerary that allows you to see a lot of the Med ports, particularly as you'll be overnighting in some of them.
What kind of information are you seeking? For example, what to do in port? Strategies for a cruise of this length? I've been to some of these ports and have also traveled on HAL (not in Europe, though), so I can help there.