I think your best bet would be Hua Hin, only about 2 to 3 hours south of Bangkok.
The beaches in Hua Hin may not be the "best" in Thailand but there are a few to choose from in that area.
I find them to be interesting, and enjoyable to walk along.
I think long,flat sandy beaches can be boring, like the ones we have in Australia.
The extra benefit of Hua Hin is the great little town to explore,should you get bored on the beach.
Also to the north of Hua Hin is Cha Am, it has a long sandy beach and good for swimming, and water sports.
I spent many an hour on that beach under the umbrellas, with the locals bringing food all day.
Or to the south of Hua Hin is Dolphin Bay, another long sandy beach but a lot less people and not much else to do apart from relaxing.
Have a great stay and enjoy the beach.