Pick up something and keep the wheels turning. Keeping fresh on news, or just pleasure reading is a skill that needs to be exercised and worked on regularly.
Depending on where you are in your academic career, one thing is fairly certain; you will be required to write the deeper you delve into subjects and years of schooling. Writing is learning on a new level.
Research something. Take it beyond Google, and do not rely on Wiki. Anyone can publish on Wiki, and some big mistakes happen there.
Research and writing work in tandem, get used to it and have fun with it, after all you will learn something this way (and far into the future).
Honing your search skill (Booleans, etc.) will pay you back exponentially one day.
- Learn something or do something new.
Find a new hobby or research (see above #3) something new or do something new after looking into it as long as it's safe of course.
- Set Attainable Goals for the next School year.
Enjoy your summer, but keep things fresh and keep working at improving. Look at getting a Tutor/Mentor/Coach to help.