Go to summer camp. A traditional, out in nature, canoes, archery, and tie dye, sleep-away summer camp. At least three weeks. Preferably, they would have started going much younger than high school, but it's never too late.
They will meet lifelong friends, have real, in-person experiences, and unplug from technology. They will come back energized, de-stressed, more confident, and with memories that will last a lifetime.