We enjoyed our time at Castaway Island. It's a nice resort far away from Nadi in the Mamanuca island group. The resort itself has a restaurant, gift shop, adult pool area (although families hang out there, which wasn't wonderful), and a kid pool area. The public/common areas of the resort is not that large.
Non-motorized water sports, like kayaks and paddle boards, are available, as well as snorkeling. There's not a large area to do these activities, given the busy water ways.
It's a beautiful island with beautiful views. You certainly get to escape Nadi and the busy port, but a downside is it feels like 2-3 hours of the 9 hour trip are spent on the ferry to/from Castaway Island from Denarau Island. Malamala Beach Club offered the same activities and amenities while only being 25 minutes from Denarau Island.
The staff on the island were friendly, but because it is an actual resort and we were on a day pass, it did feel a bit "Scarlet Letter" being the only ones wearing the colored wristband on the island.
All in all, not the worst place to visit, but there are closer, cheaper, and more relaxing islands, in our opinion.