If you have time I’d say 3 weeks. Ordinarily, tourists spend 2 weeks. When my niece came, met her in Bangkok and took her around the usual, Grand Palace, wat Arun, wat Pho. River trip to Ayuthaya for a day. Then flew to Chiang Mai, more temples , the White temple and Doi Inthanon. 2 days in Sukhothai which was the most appreciated. Flew to Phuket . Then went to Krabi and Railay beach.
Then to Koh Chang for the best swimming, islands snorkeling , great seafood on the beach, lots of long nature walks.and National park. She loved it there. I’ve spent a lot of time there, love it. And no crowds no big hotels and chill.
We went direct from there to the airport by local taxi. Great service.