First, talk to a reputable agent who's been there, done that so they can help you with the specifics and help find the best cruise for your needs, budget, and lifestyle. Never book directly with the cruise line - main reason, if you have any problems, who do you think they represent?
With that said, you'll love Venice! We've been there 3 times and it is a remarkable city. You should also do a trip over to Murano and see the glass factories.
Our all-time favorite island is Santorini! Absolutely gorgeous!!
And definitely see Dubrovnik if you can. It's definitely a diamond in the rough and a wonderful place to visit.
We spent 14 days cruising around the Eastern Med and it was awesome. Love the area!