Every summer I took band lessons. I don't remember other classes that were offered, but I think they included music, art, and fun summer reading. These courses made learning fun, gave me something to do during the summer, and helped me keep up my skills (and my skills definitely needed improving).
Of course, in high school, I took Driver's Ed during the summer. And in college, summer courses allowed me to earn my B.S. degree in three years instead of four.
Today, school district budgets often prevent schools from offering summer courses, but if you look around, you might find as school that will have something for you. In some cases, community colleges provide a wide variety of courses for students who want to keep learning during the summer, and they provide courses for all levels, K-12.