There are lots of beautiful islands in Thailand, but I can vouch that if you only had one choice, Koh Samui is a nice place to stay for several days. If you stay away from the little touristy, and NOISY, towns, you will never want to leave. The beaches are nice, the food is tasty, and it's a simply a relaxing, refreshing place.
WHAT NOT TO DO:
Don't be so trusting of the tour agencies there. Unless you have a lot of time to kill, I wouldn't take a ferry from Koh Samui back to the mainland. The tickets that promise a ferry and bus ride back are a scam! The boats are often overbooked, and the buses brutal! Flying Bangkok Airways to Koh Samui is very reasonable.
WHERE TO EAT:
Restaurants are everywhere. Lamai has some great restaurants where you can find drinks that look like Carmen Miranda's hat. I lost the business card, but In Lamai, there is a great resort that offers yummy vegetarian food, and also doubles as a Thai massage school! This place had the best spicy lemonade. It's huge, so ask around; you'll find it easily.
WHERE TO STAY:
Bungalows are everywhere, and I recommend staying away from Chewang and Lamai: you'll have peace and quiet, and your stay will be cheaper, too! We stayed at the Coral Cove Resort, which is on the east side of the island between Chewang and Lamai. The staff was friendly, the food excellent, and the beach beautiful. We lounged around every day by a huge rock that looks like an obscene phallic symbol.
WHAT TO DO:
Relax, eat, drink. If you want to go clubbing, the towns of Chaweng and Lamai have plenty of noisy clubs, filled with idiots who should not have been allowed in the country in the first place. There's plenty of shopping here as well. Koh Samui can be a pain to fly in and out of during high season. Rent a motorbike to see the island, or just ride a jeepney for a few baht; bargain for a fair price first. If you're returning to Bangkok and can't get a flight, fly to Pattaya and then take a bus to Bangkok: you'll avoid spending the day at the airport; however, if you had to be stuck at an airport, this air field is not such a bad place to wait, as it looks like a resort! If anyone wants to explain what the "See Monkey Work" shows are, I'd love to know.