Here are two suggestions
1. Arrive in Bangkok transfer to Hua Hin . About10 kms south of Hua Hin is Khao Tao which has a beautiful fishing lake, the temple complex of Khao Tao, Lion Island, Turtle Island, untouched beaches and the bays of Had Soi Noi and Had Soi Yai. You can swim and snorkel.
Do not confuse it with the diving island of Koh Ta which is 300 km south of Hua Hin
You can go by train or fly to Chiang Mai from Hua Hin and after Chiang Mai spend your last few days in Bangkok.
2. As Bangkok is full on and a big city stay there for a couple of days sightseeing , go to Chiang Mai from there fly to Hua Hin relax after your travels and then transfer to Bangkok for the trip home. (this is the one I favour)
I would not go to Koh Ta island as its too far and you can have a similar experience at Hua Hin/Khao Tao