This might sound cliche, but learn any programming language, are you into UI design? do some HTML and CSS and look into some Javascript Librarys that make nice front end websites, Like Maths and Data? Learn Python and use pandas and numPy, Matplotlib.pyplot and Seaborn and make nice data plots, freecodecamp is a great site for this or videos on youtube, not only will this allow you to add another skill but you can express other interest through coding, for visual learners you have some great videos on youtube. This is also another skill you can develop for the world of work and it shows independent study as well.
As much as people say prepare for next year in school I would probably recommend against that, summer is an opportunity to take a break from schooling and learn new things and discover new passions, there are lots of opportunities to learn new things and at your age you can afford to fail at new activities so why not try new things.