Golf, need we say anymore. The golfers will know where to go.
Cycle along the old sugar cane tracks https://www.ecotrax.com.fj/
Malamala beach club - for luxurious day trip to a very small island https://www.malamalabeachclub.com/
And snorkeling of course, offered at most resorts!
The islands are beautiful, either staying at a resort for a few nights, or a day trip from Port Denerau.