Where to Stay Ko Hai, also known as Ko Ngai, has no roads, no addresses, no stores, and no ATM's, but it does have four worthy resort-hotels right on the warm sand. The most tasteful are basic—the rooms have air-conditioning, small refrigerators, and bathrooms whose showers are open to the sky. Spend a night at Coco Cottage (66-8/9724-9225; coco-cottage.com; doubles from $114) in an eco-friendly wooden bungalow or at Thapwarin Resort (66-7/521-8153; thapwarin.com; doubles from $97), where interior walls of woven rattan, vertically striped bamboo exteriors, and coconut roofs (like shaggy toupees) give an immediate frisson of aesthetic pleasure. Koh Hai Fantasy Resort (66-2/316-3577; kohhai.com; doubles from $168) has a maze of cottages with small gardens and ponds (occasionally loud with frogs), a candlelit Thai spa, a large tiled swimming pool with an adjacent snack bar, and a staff that coordinates numerous boat and scuba trips. Another option is the larger Koh Ngai Resort (66-7/520-6924; kohngairesort.com; doubles from $152), with modern cottages on the beach and older apartments in contemporary wooden Thai houses, plus a patently perfect crescent cove of beach, though it is a bit isolated (a 20-minute walk to the other resorts). On Ko Muk, a more remote island 30 minutes away by boat, the Koh Mook Sivalai (66-89/723-3355; komooksivalai.com; doubles from $217) sits on a long spit of beach; rooms have clean lines, glass doors, and beds that look out over the azure sea.
Where to Eat Food-wise, Thailand is the Italy of Asia: the possibility of getting bad Thai food on Ko Hai is genuinely unlikely. The only stand-alone restaurant on the island is a funky and nameless barbecued-fish spot a few minutes' walk north on the beach from Koh Hai Fantasy Resort. It's a must, however, as it offers crusted whole snapper (of the-one-that-got-away size). Pair the shockingly hot small Thai chilis with a cold Singha beer. At Koh Ngai Resort, choose the creamy massaman curry with cinnamon and cardamom. Order the country's ubiquitous som tam (a salad of shredded green papaya, carrots, cashews, and baby shrimp, zinging with strong-guy chili-lime dressing) at Koh Hai Fantasy, then let the staff grill up whatever they've caught that day—it could be barracuda.
What to do The beach is the biggest attraction on Ko Hai, as the island and the sea beyond are protected by the government (the 83-square-mile region is a National Marine Park and is home to more than 100 species of coral fish). Reefs ring the island, which makes for astonishing snorkeling: an excursion on a long-tailed boat to Emerald Cave on Ko Muk is mandatory. Swim through the 260-foot-long cave (really a tunnel), plunge briefly into primal darkness, and emerge, suddenly reborn, on a perfect small sandy beach surrounded by impressive cliffs sprouting jungle vegetation. Arrange a full-day scuba trip in deep water with Rainbow Divers (rainbow-divers.com; full-day trips from $33) to spot sea tortoises, whale sharks, and sea horses. After a morning in the Andaman, opt for a Thai massage under the coconut palms at Thapwarin Resort. Later, try to spot a long-billed hornbill (hint: look toward the palms around the swimming pool of Koh Ngai Resort). But the best option may be to do nothing at all: find a place on the beach and relish the vista of islands—their vertical rock cliffs like limestone candles pop straight out of the sea.