My 2 kids tell me the best cruise for them was the 14 day Mediterranean cruise on Norwegian Jade ship in 2008. They were ages 16 & 14 them and got to visit London, Southampton, Lisbon, Monaco, Monte Carlo, Tower of Pisa, Rome, Vigo. And they made friends from many countries in Europe, some of which they still communicate with! Overall it was way cheaper than traveling in Europe on our own.
I would love to do a cruise like that but I have difficult getting the wife to take anything that is longer than seven days.