Many of the River hotels in Bangkok would allow for what you describe. The Peninsula, Hilton, Shangri-La, Sheraton, etc... (in fact most of the 4 & 5 diamond properties would allow for a relaxing experience) I love Bangkok. The river seems tailor made for the 1st time visitor.

Hua Hin is OK, it has the ocean and sitting under a canopy of hundreds of beach umbrellas eating seafood is a great experience. Six Senses has a great reputaion. But no shortage of great hotels in all areas of the country.

Why not do both.