I have no advice to offer on a med cruise per se.
When my kids took their first cruise ~ my son was 2.5 and my daughter had just turned 4! They loved it! They keep track and refer to things that happened on their #1cruise, their #2 cruise .... this summer they are going on their #3 cruise and they are stoked!
I also disagree with Hawk-eye on Disney.
Our first trip to Disney and my son was 18 months, and my daughter was almost 3. What a way to experience the MAGIC of disney. I will say that we planned the trip to make sure the schedule and activities were appropriate. My daughter was a bit scared of the costumed characters (like Mickey). My son was in AWE. I have memories of doing the Character Spot at EPCOT and my son was just giggling .....he he, hee, hee, heeee heee heee (he was doing a pretty good beevis and butthead impression!). When my daughter met the princesses, she was just IN LOVE! It was magical. I printed out "invitations' inviting them to the character meals. The invitations would appear while we were out at the parks, and when we returned they were both sooooo excited to find out where we had been invited next. My daughter wore all of her princess gowns. I remember one night she was frantic because she was dressed in her Cinderella gown as we were off to 1900 Park Fare .... and she was frantically trying to wake her dad because she was afraid she would be late for the ball! Everywhere she went, she was called PRINCESS, and yes, she truly believed she was one! We stayed at Wilderness Lodge, and everytime my son saw the totem poll, he would start shrieking MICKEY, MICKEY, MICKEY! They loved every minute of disney from pin collecting to rides, to character meals, it truly was magic for them .... and neither had turned 3.
....ahhhhh..... good memories!