I was just on the 5-night Fantasy that had 2 sea days and stopped at both Lookout and Castaway.
Looking back on my phone, these were the number of steps it said I recorded:
Embarkation Day (11/16): 12,885 [this one is skewed since we went to
Jollywood on 11/15 and that ends at 12:30am]
Day at Sea (11/17): 10,692
Lookout Cay (11/18): 11,158
Day at Sea (11/19): 7,432
Castaway Cay (11/20): 7,905
I would say it depends on what your preference in activities are. On Lookout, there's the pier that you have to walk down to consider. They do offer a shuttle if needing assistance but they say that only the person needing assistance can use it, everyone else should walk to the beach. My dad had used it, he had some blood blisters on the bottom of his feet, and he said that it didn't take him long to be able to get on a tram and he was actually able to meet us at the second tram to head to the beach. We arrived at the same time. I also walked around on the island to find seats as well.
On Castaway, it's not as long of a walk to get to the beaches and the day was mostly sitting in the loungers whereas I was more inclined to look around Lookout since it was my first time there. My first day at sea was more of a pool day, but we met a lot of characters, did the Midship Detective Agency, and my family will choose to do the stairs if the line for the elevators is long.
