Take Advantage of the Nursery - As you can imagine, Disney does kids clubs in a truly amazing sense. Brody was only 17 months on our trip so the only kids "club" he could be in by himself was the Nursery. On the Fantasy, It's a Small World Nursery was for kids from 0-3 years old. Disney charges $6 per hour and recommends booking your time ahead as space is limited (although we never had a problem taking Brody spur of the moment the two times we needed to). We used the Nursery 3 hours each of our "at sea" days during Brody's nap time. They gave him snack, put him in a crib for a nap, and then played with him when he woke up. This was great for Dan and I so that we could enjoy some of the "adults only" areas on the ship - like the Quiet Cove pool.
Check out the Oceaneer's Club Open House - Even though Brody was too young to be checked into the Oceaneer's Club during the day, he could still attend the daily open house hours with Dan and I. This was usually in the morning and allowed children to explore the space with their parents. It's a great chance for the younger toddler crowd to get to try out the "big kid" play areas and also get visits from Mickey and Minnie for play time. Brody especially enjoyed playing with all the Toy Story figures in Andy's Room.
Pay Attention to Toddler Time Activities - DCL has a special listing of activities that they give to families cruising with toddlers. These include highlights during the sailing of things like Wake Up with Disney Junior and Jake and the Neverland Pirates as well as toddler playtime in the D Lounge. We went to several of these activities and enjoyed each. Most are scheduled in the morning because Disney understands that little ones don't know what a vacation is and continue to get up early. The songs and character visits were fun and familiar to the kids and the shows were all designed to be interactive.
Nemo's Reef - If you are on a cruise with a toddler, they won't be able to use any of the pools. Maritime law prohibits kids who aren't potty trained from using a pool on a cruise ship so Disney created a splash and play area with toddlers in mind. I wish Disney enforced that this was designed for kids 6 and under because there were a lot of upper elementary school aged kids messing around in here and playing kind of rough with the little ones so close by. The theming is adorable and all of the slides and water features are made to be toddler friendly.
Dinner & Shows - Every kid is different so this last one may not apply to you, but Brody is a toddler who needs his schedule. Having first seating for dinner was really important to us because his normal bedtime was 7:30. We had dinner each night at the 5:45 seating and saw the 8:00 show. There were several nights that Brody couldn't make the show so either Dan or I went back to the room to put him down for the night. I learned about halfway through the cruise the DCL plays the show on their stateroom TVs at the same time it's being shown in the theatre so if you can't make it down to the show you can still catch it in your room. We took advantage of this a few times on the cruise which was a nice feature.
Cruising with a toddler can really be a vacation for all involved if you learn some of the ins and outs of navigating the Disney cruise ships!