If you're religious, you might want to try summer Bible School. Many churches offer one or two weeks of religious training in the summer. It's not formal study, but is usually quite informal and usually a lot of fun. You can keep up on your school skills because Bible School offers many of the same types of learning you get during the school year.
You will get to read stories and listen to them. You get to play games, do skits, sing songs, work on projects, and many other fun things that will help keep your brain active.