We (adults 50's world trvelers fromUS) went to Thailand once 2 yrs ago--BKK is doable in a few days. We lvoed even one full day in Kanchanaburi (once we found out how to pronounce it, we'll be back!!)
Hua Hin was the most beachy location we'd chose to visit--it's just not an attraction to us, like Phuket, islands etc are to Aussies. our objective was travel not vacation, so it may not apply to you??next trip--repeat Kanchanaburi and much farther north for more culture/history.