Other kids did summer classes for credit at the local high school. Albegra, Geomtery, Chemistry. These was intense with many hours of work being done daily as the full years curriculum had to be covered in a short period of time. There was homework, quizzes, tests and grades. With the grade appearing on the transcript.
My son did a non credit program for two weeks in England last year and three week language immersion program in Germany. None of it was for credit. Both were largely for fun. Little studying happened in these programs. I sent him largely to experience living abroad and become independent. And network with other kids that have similar interests.