The pressure can be high on kids to be productive in the summer, and to make the most of their free time. One can understand where such expectations come from -- after all, summer is also prime time for the feared "achievement gap" to sneak in between kids who do nothing academically-stimulating, and those who continue their educational pursuits. However, you don't have to go to an exotic summer camp to learn new things; you don't even have to leave your house.
What I suggest is not limited to the simple adage that reading a book will take you to new worlds (though that is absolutely true). Rather, I encourage kids of all ages to see learning as a state of mind: if you are looking to learn, then you can cultivate your mind almost anywhere, doing almost anything. Books are in fact a great place to start: try the thirty-second exercise of thinking of something which fascinates you and doing a google search for books on that subject. No matter what you love, books can make more out of that passion in an educational and enriching way.
Yet why ought one to stop at books? A computer gives you immediate access to a plethora of information and knowledge. Of course, it is important to note the difference between these two things: not all information online is knowledge. In fact, not all of it is correct. Reading scholarly articles and looking at material posted by educational institutions (those with ".edu" domains) are the most reliable sources of the knowledge you're looking for, but that's only a place to start. Being bold and reading up on less-reliable, online forums or other websites can sometimes be useful as well -- particularly when you fact-check the information against more reliable sources. I learned a lot about bodybuilding by browsing tnation.com postings, trying out their workout methodologies for myself, and reading up on the science behind exercise to see what really works. The internet can be a powerful tool for learning when you use it well and responsibly.
Even the most remote places, where one would expect to only find pure leisure, can become educational exploits when viewed through the lens of one hungry for knowledge. In high school, I designed a course of independent study in which I critically analyzed the role playing dynamics of video games at a granular level. A well-constructed video game can be as breathtaking and enriching as the most elaborate novel. Mind you, I'm not talking thinking of something like "Call of Duty"; but role-playing games often contain epic stories spanning entire worlds, following characters which arrest you with their conflicts and convictions, and which challenge your core philosophy. Of course, you can also play a game mindlessly, just as you can mindlessly read a book. The key is to be mindful of what you are experiencing, ask critical questions of it, and remain open to the opportunity to learn from it.
So journey to different worlds this summer, even if you stay at home! If you are home alone a lot, learn to cook some new dishes, because cooking will never go out of style. Nothing is done that cannot be done well, so welcome in the experiences of your day, and let them educate you!