If you like snorkeling, Mare and Mystery island have some fantastic reefs to explore. Isle of pines has amazing sea snakes.
If you have a diving ticket, Espiritu Santo, Vanauatu has some of the best diving with the SS President Coolidge ocean liner turned WWII troop carrier wreck in only 20m of water just off the beach and Million Dollar Point where the Yanks dumped all their arms and supplies at the end of WWII from tanks, jeeps and trucks to simple helmets, plates and cutlery. Being a bit of a history buff I really enjoyed both these dives.
Also the Zego self drive jet boat tours in Port Vila are great fun. Beware the mad Taxi drivers though when cruise ships are in port. They literally fight each other to get your business.