Volunteer for Jungle Aid, PAWS, Trash Heroes Beach Cleaning, Royal Rotary Club Hua Hin, Hua Hin Heroes. No work permit. No problem and don't let the fear mongers tell you differently. They are all protected.
Don't sweat it...the percentage that Immigration takes the time and effort to investigate, apprehend, process and fine a foreigner for volunteering (normally to help Thai people) is .005%