If you are on summer break from high school or college your best bet is to get a job. Even if the job is not in the area of expertise you are studying. If you are going to be an engineer and end up working retail for the summer think of all the additional skills you will learn. Not only does it pay you but it is golden on a resume. It shows initiative and that is what prospective employers want in a candidate.
So you say you cannot find a job in this economy? Volunteer!! This is also a great example of giving your time unselfishly to help others. Personally I love to see this on a resume of someone just out of high school or college. There are so many places that need volunteers and you will gain a lot of good experience just as if you were being paid.
Last but not least look for an internship in a company that has the potential to hire you down the road. You should try to start in your sophomore year and go back every summer if you can. If you want to do this then you need to apply probably 6 to 8 months beforehand and most companies have a site for intern applications.
In this day and age there is no time to sit back as when you leave high school or college to enter the workforce this is what a recruiter or hiring manager finds impressive. Working and gaining business experience during your summer break will put you ahead of the rest.