Get a job. Colleges like to see that you work and are saving some money. A common question on college applications is 'What did you do with the money you earned?'
Take a summer program at a local college or one in a different part of the country. This can be a good opportunity to explore a major you are interested in. Many colleges like to see that you have explored a career interest.
Start searching for a paid or unpaid internship. Many are posted now. Try Internships.com, Looksharp.com or Indeed.com or focus in on your major interest. It it's drama, look at a local playhouse, or if it's science a local university that is strong in science.