Learn a new language over the summer. If there is a community college or other higher-education institutions near your town or city, visit their websites to learn more about their summer language learning opportunities. You can also embark on a self-taught language learning project with the aid of online or CD-ROM courses that will help you practice pronunciation, grammar, and spelling.
Put your idle knitting needles to work in a summer crafting course. If you've always wanted to learn how to make pottery, find a local pottery course over the summer. These summer craft courses are available in most cities in the United States for youth and adults. To find out what your town offers, visit your town website or look for adult learning opportunities in the local newspaper. Many cities also publish a catalog of summer course offerings in everything from ballet to baking to carpentry.
Learn to swim over the summer--- the season makes it the ideal time! Whether at your local pool or town pond, there are usually individual swimming lessons and group swim classes available around the country during the summer. Swimming is fun, good exercise, and a great life skill to have.
One of the characteristics of summer is the increased downtime: school is out, and workplaces typically scale back their hours or are otherwise less intense. Use that additional time to your advantage and make a dent in your reading list. There is likely a list of novels you've always meant to read but haven't ever got around to. Consider starting a book group with your friends to keep one another motivated and to keep up the momentum for your summer reading challenge.