Speaking from experience, Mediterranean cruises are so wonderful (and romantic). They are rich in history in culture and there is so much to do! They mostly seem to be split into two categories, Western or Eastern Mediterranean. I rarely have seen ones that verge somewhere in the middle, and many itineraries are much the same as the other, so let me go over some of my personal recommendations.
I would recommend RC and Norwegian from research considering ratings and price combined, and Disney from personal experience. There is SO much to do on Disney ships and believe me, it is not all kids. The entire experience is just as luxurious and they run a very tight and beautiful ship.
Most itineraries seem to be split up into a few categories when it comes to mediterranean cruises. You have Western Med (anything from Spain, Southern France, and Western Italy), Mid-Med (Eastern Italy, Croatia and partial Greece even maybe Kusadasi), Greek Aisles (just like it says, sometimes including Turkey), or Holy Land (usually a mix of cities from Italy, Greece, Turkey, Israel, and Egypt).
I have been wanting to do another Mediterranean cruise as well, and I am personally looking at doing something Mid-Mediterranean for my boyfriend and I. I loved Venice last time I went and if you can pick up a cruise with Venice at the end or beginning, you have tons of possibilities of travel added onto the cruise. A standard train ticket (clean and about 4 hours long) is not too expensive to take you into Rome, and you can get 24-hour tour bus ticket things for like $25 that will take you around all the sites if you haven't been there before. Alternatively, you can easily get to Northern Italy like Milan or even Southern Switzerland (maybe even as far as Basil) from Venice. Traveling is very easy and there are so many english speaking citizens.
I also LOVE Greece, and no matter what state they are in, they still appreciate the tourism. You don't have to travel to the parliament area in Athens if you don't want to (not much there). Just stick to things like the Parthenon and Plaka, and some of the isles. I highly recommend Santorini -- my most beloved vacation spot in the world so far. Extremely romantic, and while people think it's all pricey, you can definitely get by on a serious budget.
So that's my recommendation. Try to find a shorter cruise that visits Greece and Venice, then hit up another city or two near Venice at the end. You can PM me if you have any questions on how to get around! I've done (almost) it all.