I remember Thung Wua Laen Beach, about 15 km from Chumpon - nice clean water, cheap bungalows for 400-500 Baht. Not busy, not many foreigners, only thais on the weekend. Not too developed when I was there a few years ago; hope that hasn't changed much.
It was my "secret getway" place, I lived 100km north for about 7 years.
Only drawback were the sandflies on the beach!