Apart from the popular Bangkok, Phuket, and Pattaya, here are some of the most must-visit destinations in Thailand:
• Chiang Mai
• Khao Sok National Park
• Chiang Rai
Start with Phuket and Krabi are similar.. hardly 2 hours away from each other. Try to stay in Koh Lanta ( 1 hr from Krabi ) for peaceful and beautiful beaches. Chiang Mai has beautiful mountain views , Samui is totally separate from the Land. You can only go there by flight or Ferry from Suratthani and bit costly flights. I'd Suggest 3 Days in Krabi , 1 Phi Phi , 2 Phuket , 3 Bangkok
Koh Samui - you will get island tour from there ..Koh tao and nang yuan island tour. These islands are really good for snorkelling.
Phi-Phi Islands - must book the maya bay Sleepaboard from there: https://www.mayabaytours.com/
This is one of best experience I had while i was there.
Krabi - You can book the 7 island or 4 island tour. Phuket - Patong beach
If you want to visit the already popular places like Pataya, Phuket and Krabi then do know what all pics you see in the internet won't be similar to the real place. So I would suggest try some offbeat places there like phungsu, phi phi.