For the beach, I would recommend the neighboring island of Koh Chang,
Ko Maak . Ko Maak is quiet and not very developed. White sandy beaches, clear, warm water. You can hike in the jungle and even find a deserted beach or three

With five days, yes you are pretty limited. I would third Kanchanaburi without a doubt as well as Ayuttaya. If there was time, I'd also recommend visiting
Sangkhlaburi, which is quite lovely and a very culturally diverse and colorful, picturesque town.
have a wonderful time
