Mana island is recommended. Very serene. You can get there quite easily from nadi. Even riding on the boats around is nice. If I had the choice I would stay more in places like mana vs something like sheraton denarau
It has been a long time since I've been there. The main attractions are the beaches. The water is gorgeous. The sand is beautiful. It really is very tranquil and relaxing. You can snorkel, scuba, take a boat to various secluded islands. There are various Polynesian island cultural things that you can visit though these are often kind of typical tourist fare such as hula dancing etc. Lawn bowling was really popular when I was there and it was a lot of fun. There were also grass tennis courts on which I was able to play if that's your thing.