Get a job (EVEN if your last name is Gates or Trump and you don't have to). Check out http://www.jobmonkey.com/ for some summer job opportunities.
Volunteer in your community. Summer is the perfect time to seek out new experiences. Some students enroll in university programs for a taste of college life.. Others embark on archaeological digs or bike tours. Find a passion to pursue!
Get involved in extracurricular activities
"Job shadow" someone who does what you think you'd like to do (that means to follow them around for a day)
Attend the CLHS college fair in September
Start a college savings account and regularly deposit into it (in addition to what your parents have for you)
Explore careers and job opportunities in those careers. Check out these websites with quizzes and ideas for future careers.