There are a lot of interesting things in Thailand. Of course, there are many beautiful beaches in Thailand, but you should not spend your entire vacation on the beach and in the hotel. Travel on your own or book a guided tour. Get to see more during your vacation.
You can probably buy a group tour in Thailand after arriving at the place. They are offered a lot in any tourist place: Pattaya, Phuket, Koh Samui, Bangkok. There is no need to book a group tour in advance. (Unless it's some sort of long-distance bus journey. Seats are limited on the bus.)
But if you need something special, an individual program, personal service, then a personal guide will help you. You can order an excursion with a personal guide in Phuket, Pattaya, Koh Samui and Bangkok in advance, online.