It isn't always necessary to cram textbooks during the summer; try to find simple ways to maintain an overall positive summer in terms of learning by using simple steps: with just a few easy habits, you can reinforce what you've learned in most mainstream classes without feeling like you're under your teacher's thumb!
Try typing with proper grammar/punctuation when texting, and using IMs or Facebook. This will get you into the habit of knowing when to use correct capitalization, avoiding word mix-ups such as your/you're and there/their/they're (which are surprisingly common in people who should CERTAINLY know the difference); when using an actual keyboard, this will slowly increase your typing speed, as well. I started doing this myself in 9th grade at 13 years old, have never had a message misinterpreted due to "i cdnt rd wt u sed", and have since greatly improved my typing speed, so much so as to be faster than some people I know who type without looking at the keyboard! (Yes, I look at the keyboard while I type; not every English teacher is a typing prodigy!)
For mathematics, use your head for simple math problems, such as determining tips at a restaurant or calculating change, figuring out tax on items you buy from the store, and other commonly encountered situations. (Hint: memorizing your multiplication tables up through at least 12 is a big help quite often in life, so I'd recommend learning them if you haven't already!) In addition, if you are highly skilled in math, consider picking up a tutoring job through WyzAnt or from a local business, or simply put up your own flyers around your city! Who knows? Perhaps you'll end up having a little extra income over the summer, and all the while you'll be keeping your mind in ship shape!
Try practicing your foreign languages with other people you know who speak the same language. If you work at a place with people who speak more than one language, try conversing with them! Even if you aren't very good, you learn a language MUCH more efficiently when speaking it and having it spoken to you than by researching it in a book and seeing words on a page. If you're interested in more practice, try finding friends online who speak other languages, and get in touch with them! Learning a language is also a WONDERFUL trait to have when looking for jobs, especially Spanish! If you're even moderately comfortable with your language, put it on an application! It won't hurt!
Here's to hoping you have a great summer, filled with ever-expanding and reinforcing knowledge!