If you want to visit a few places then Cha-am, Hua Hin, Prachuap Khiri Khan, Ban Krut, Bang Saphan, Chumphon can all be visited by train at low cost. Going to multiple islands is more difficult.
Hard to find good Thai food on islands apart from Phuket, Samui and Chang. The minor islands aren't known for good food and mostly cater to western taste which means bland Pad Thai and things like that.
I guess you have not been to many islands. A lot of them have rubbish collecting on the beaches when it is not busy. Most of the beaches are not that good anyway and food is usually disappointing.
Tamarind night market is the best one for food. The main night market near the train station has nowhere to sit apart from overpriced restaurants nearby. Baan Khun Por would be the second best night market.
I visited Khao San last month. It wasn't very busy. I stayed about 20 minutes north of there and hardly saw any tourists. In Hua Hin I only saw lots of tourists in the night markets. Many of the beaches there were hardly any. Mid year Thailand is quiet mostly outside a few popular spots. In Bangkok it is so easy to avoid tourists. Anywhere from 500m north of Khao San to Don Muang you won't see many at all.
Islands are usually the busiest in high season and beaches loaded with rubbish in low season as nobody wants to clean them. Also far more expensive and food is normally not good.
Well give it a try. I recommend Sombat buses. Go to the beach opposite Market Village or south of there. Find a hotel near Market Village. If you like gyms go to King of Muay Thai.
Samui is a lot more busy than Hua Hin and Samui is more expensive. I doubt Samui has rooms with pool and gym under 500 baht. I have never seen that value on any island. Never seen it anywhere apart from Hua Hin.
You watch $1 movies instead of going to the beach and have not visited Thailand in years have you not? What year did you last visit Hua Hin or any other Thai beach?
Hua Hin is better now than ever before. There are more affordable hotels and the bypass road takes traffic away from town. I even found a condo with daily rates under 500 baht with pool and gym, steakhouse next door. The condo had a convenience store and a 7/11 vending machine.
If you haven't been there for many years which clearly you haven't saying it is a rubbish town is just ignorance. I found a gym for 890 baht a month, better than any gym I have seen elsewhere for that price.
I visited all the beaches within 20km of the centre. The only bad section was north which disappears at high tide which was basically under soi 51. Above soi 61 the beach is very nice for swimming or walking. The next 2 bays have food sections.
The foodcourt in Market Village has 25 baht shakes, cheap meals and export shop for clothes.
That isn't true. I doubt you have been there for years. The only part that disappears at high tide is the northern part. South of the train station the beach is great and extends for several kms.
You probably went there 15 years ago and only visited the northern part. I was on the beach in May this year. It was great. The horses are only in one small part. The water quality was also good. The beaches further south are no better at all. Ao Manao water is less clear and more shallow.
In 15 years everywhere north of Bangkok will have severe pollution for 3 months a year. I see no improvements and it will get worse. That makes everywhere south attractive and Hua Hin is close to Bangkok if you want the best hospitals. I can only see Hua Hin improve.