Hot Take: Lookout > Castaway
I know this will be a minority opinion, but to me Lookout Cay is far superior to Castaway Cay.
The beaches at Lookout at perhaps the most beautiful beaches I have ever seen in my life. Castaway's beaches are very nice, but nothing extraordinary.
Lookout feels more spread out, with more room. Castaway feels crowded, full, and can be rather loud.
Lookout's bathrooms, shops, etc are all exceptionally clean (they are also newer, so that may be why). Castaway again feels like a more lived in island that has seen a lot of guests come through.
I do think I'd give a slight edge to Castaway for their food, but not by much. The kid's splash pad area on Lookout is incredible.
All those ramblings aside- Castaway gets a lot of love for the nostalgia, and the feeling of it being a cozy Disney island. Lookout gets a lot of hate because of a long bridge. If I were on a cruise that went to both, and could only get off one of the days- I'd go to Lookout 10/10 days.

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