Be sure to enjoy plenty of cheap, authentic food back in Trang or Krabi, because you won't find much of it on Ko Ngai, where a bottle of water can cost more than a meal on the mainland. Expect to pay a minimum of 100 baht for simple dishes like fried rice, with curries and stir-fries running between 120 and 200 baht, and seafood, along with Western dishes, hitting 200 baht or more. No standalone restaurants or bars are found on Ngai, so you're at the mercy of the resorts.
Remember: if you spend a small fortune on a terrible meal, remain calm and refrain from shouting or otherwise making a fool out of your self. Breathe deeply, go for a walk on the beach and grab a Snickers bar or bag of chips from the minimart at Koh Hai Fantasy. You'll live to eat another day.
To ensure that the above scenario doesn't have to unfold, pull up a chair at one of the tables scattered haphazardly over the sand at Koh Hai Camping. The cook seems to understand what the word "spicy" means, even when spoken from a Westerner's lips. They do a standard menu of Thai dishes served with plenty of flavour, and in large portions, at more reasonable prices than elsewhere. Their nightly barbecue also competes with Koh Hai Seafood down the beach.
For more of an upscale dinner in a romantic setting, Thapwarin Resort is a fine choice. Western dishes like French onion soup and beef tenderloin join Thai curries and other dishes that are well balanced despite being toned down for Western tastes. They also offer an extensive seafood barbecue menu along with all kinds of iced coffee, teas, mixed juices and alcoholic cocktails. A midday spicy ginger soda is a good choice, or perhaps the house mixture of rums and juices called "Bumbastic!" for after hours. At the opposite end of the beach, Thanya Resort is another solid upscale option with dishes like roast duck and a range of Thai-style salads.
Ko Ngai is far from being a party island, but the beach bars at Koh Hai Camping and Koh Hai Villa can both be fun places to chat up other travellers. A pricier option would be Koh Hai Fantasy, which serves mojitos and pina coladas in whole hollowed-out pineapples at discounted prices from 12:00 to 20:00. The large groups of Europeans often staying here can get raucous on the beach decks after dark, making it easier to let loose than at other upscale resorts.