Summer is a crucial time for students to continue their education. I know they have just finished up the school year and would love a break, but there needs to be some activity from the students. An easy way to keep your brain going is to actually study things you enjoy. Instead of sitting at home and going through your basic classes (Math, English, Science, etc), I suggest incorporating something you enjoy or find interest in. If you already enjoy a hobby then expand your knowledge in that field. Learning can take place in all fields. For instance, I have been wanting to learn more about light waves and solar energy, so I signed up for free classes on Coursera.com for the summer. They are basic courses with a light load of work, but I will most likely put more effort into these courses because I actually find them interesting.
Another excellent solution is games. Games are a great way to hold your students undivided attention with fun. There are a bunch of brain games out there and puzzle games that are great to keep the mind working. One of my favorites is Lumosity. They incorporate brain games with competition, so there is an incentive to become more skilled in the games. A friendly competition is always a great way to effortlessly stimulate the mind.