Friends trip slowly turning solo, where to go

Hi, I have been to thailand a few times before but only to BKK and Pattaya. Im flying on 20th March for a 10 day trip. Wanted to do a bit of exploring towards the islands and was planning to visit Koh Samui this time but my friends have started to drop out. Im now in a fix about what to do and where to go. Need tips about a couple of things :-

• Whats better to visit and stay for a solo traveller - Koh Samui or Koh Phangan, Flights to samui are quite expensive so would it make more sense to fly to Surat Thani and take ferry ?
• How many days do I need to visit Khao Sok National Park ? Any good guided tours, that can help me cover some of the bits ?
• Is it a wise idea to fly out of Krabi airport for international flight ?

What would be my best itinerary to visit Koh Samui/Phangan, Khao Sok and then fly out of Krabi ?