My (at the time) 3 1/2 year old loved the morning "Wake up With Disney Jr." events on the Fantasy (basically, dancing around in one of the lounges while they played music from the various Disney Jr. cartoon shows (Doc McStuffins, Jake and the Pirates, Sofia, etc.)), although Dad and Mom weren't exactly thrilled with the usual 8:30 am starting time. Every night, either before or after dinner, there were "Family Dance Parties" (usually in the Tube) with lots of little ones jumping around to (mostly current, but Disney-approved) pop music, always had a few of the Disney counsellor-types to show line dances for the older ones, and often a special "unannounced" visit from Goofy or Pluto (on my Fantasy B2B cruises, Goofy had some excellent moves...Pluto was more in the "left shark" category).
Be sure to sign up for the character breakfast (again, usually Disney Junior characters, plus one or two from the "regular" gang); I think you can do that in your online check-in now (we had to wait in line the first day at Guest Services for tickets).
Our little one also loved the splash zone, but note that
DCL will NOT allow kids with swim diapers in the pools, and they STRICTLY enforce, so until they're potty trained, the splash zone may be her favorite spot on the ship. Character visits can be fun, but the waiting in line was an issue with my little one (although the lines were MUCH shorter than in the parks, and there are many more opportunities for chance encounters with the characters while walking the decks, etc., then you'd ever see at WDW). And every kid reacts to characters differently...on our first DCL cruise, our DD loved the "head" characters, but absolutely hated the "face" characters (Princesses, Peter Pan, etc.)...you probably have a good idea about that from your WDW trip.
Last but not least, enjoy and be flexible...there were days when all my little one wanted to do was spend an hour climbing around the chairs in a vacant/unused bar/club, so organized events took a breather during those times. So long as she was happy, so were mommy and daddy.
Enjoy!!