Hua Hin has flights to Chiang Mai and air quality and beaches are good. Two major malls, lots of condos with gyms and good stand alone gyms. King of Muay Thai gym is great. It has crossfit, strongman as well as weights and Muay Thai.
Prachuap and Ban Krut are a little boring after a few days with no major malls and limited restaurants and condos.
Trat is a small town so would also be boring.