May is an ideal time to cruise the Eastern Mediterranean - it becomes VERY hot and quite crowded in the summer. Your big decision, sticking to Noordam, is if you want to sail the Eastern Mediterranean or the Western itinerary - both cover the high spots.
I, personally, love Ephesus (Kusadasi), Corfu, Santorini and Messina, especially if the latter offers you a tour to Taormina. It goes without saying that Rome is fabulous, and time permitting, you should factor in enough time to include several days there.
If Northern Europe interests you, then May is a bit early as the temperatures can be quite cool. June/July would be better "up North".