Espiritu Santo (also just Santo) is in the largest island in Vanuatu.
Espiritu Santo is a diver's paradise with a multitude of sites and wrecks. Its most famous wreck, the President Coolidge, is known for being the largest accessible wreck dive in the world. It is a fascinating ship with a dual history as a luxury cruise liner and a military transport vessel and the wreck retains evidence of both lives. It requires experience experience with deep dives and wreck penetration to safely explore. The wreck is so large that there is anecdotal evidence of divers getting lost inside and having to rely on strategically placed emergency oxygen bottles to escape.
Another famous (and less intimidating) dive site is Million Dollar Point where the United States military dumped all of its equipment into the sea after WWII. Divers and snorklers can enjoy exploring sunked tanks, jeeps, aircraft and millions of 1940s coke bottles.
Millennium Cave is another attraction. Only found in 2000, tourists can make the trek to this large bat-filled cave and then relax while tubing back to their starting point along a lazy river. Wear old clothing, sturdy shoes, and realize that there is an active bat colony which will crap on you. Remember that you can wash back at your bungalow and enjoy the experience.
Trekkers and nature walkers have numerous options, including several multiday treks. Ask around in Luganville for tour operators and guides.
Champagne Beach is often quoted as being the most beautiful beach in the South Pacific. Watch out since it gets invaded when the cruise ships come to call. There are many other beautiful and isolated beaches. Sharks have been known to attack swimmers and there are some dangerous currents so be sure to ask the locals before jumping in.