Southern Thailand has a great variety of things, from beautilful tropical islands to the oldest rain forest in the world.
Most holiday makers will acknowledge Phuket which has grown quickly over the last decade and one of Thailand's popular tourist destinations. Although this could be the most popular destination in southern Thailand, it is far from being the best place to go on vacation in the area.Krabi which is a bit due east by Phuket and is a much cleaner version of Phuket, prices are about the same but the quality of just about everything is better apart from the nightlife.
If you go further down south of Phuket their is Trang, if your looking for beautiful islands and great snorkeling at half the price as Phuket and also much better tours. Trang is your best bet, I heard about this place through some Thai friends and was very impressed.
Then if you go a little north of Phuket you will find the oldest rainforest in the world in Khao Sok. This place is one of my favourite places in Thailand, the natural wildlife and jungle is just beautiful. If yu want to get closer to nature and relieve some stress, a week in a tree house in lush green Khao Sok is the way to go. I have no idea why this place does not get more tourists.
Their is other gems in southern Thailand but Trang and Khao Sok are my 2 favourite destinations and would recommend it to anyone. Their is still much more to discover in southern Thailand I hope to do so as soon as possible.