I lived in the South for over 20 years and traveled all over so I can give you a few ideas, Pretty well the best attractions are coastal, beaches & islands, though there are some nice waterfalls and caves around the interior. From HatYai you could head to Pakbara in Satun province and from there take the boats out to islands: Tarutao, Lipe, Adang. Next Province up is Trang (more islands) then Krabi (more islands) You can island hop up that entire western coastline (Andaman Sea side). Alternatively, you can go to Songkhla, then move up the eastern coast (Gulf side) to Koh Samui/Koh Phangan. If you're more into islands/beaches then the Andaman side has more to offer. You also need to consider time of year, as the weather patterns are different for either side. You really need to get hold of a good guide book, something like the Lonely Planet, that will give you all the details and recommend places to stay. You can also look at a map of the South and work out which places catch your attention, then look on YouTube for trip reports, they usually go into great details regarding places to stay, interesting restaurants and accommodation.