Working over the summer is probably one of the best things you can do with your time. In high school, friends would complain when they would work over the summer instead of doing some prestigious internship. To colleges, having a job and working is more indicative of the person you are than some 2 month internship. If you are someone who will be relying on a university to give you sufficient financial aid, having a history of working over the summer helps you to create a record of being someone who is actively trying to make money. It certainly helps to have a little extra cash when it's time to pay for standardized tests or college applications whose costs add up pretty quickly. Having applicable work experiences is also valuable for resume creation among other things. In general it is just good to have work experience under your belt.
Doing an internship over the summer is also a really great opportunity to explore career or major paths. Working in an internship is also another way to build your resume and to build contacts in a field. Often you will find that there are other similarly aged interns working beside you. The best part about internships is that it does not necessarily matter what type of internship you do as you are not locked into that field or study. For example, though I was interested in studying Business in college, the summer before I went to college I did a Congressional internship with a local Congressman. Certain internships are generally well rounded enough that you still gain a lot of valuable skills from working in these environments. Like having a job, internships are a good way to get good letters of recommendation. However it is important to note that the value of an internship really comes from what you make it.
Summer is a great opportunity to get started on applying for scholarships. During the two summers before college, I set the goal of applying to at least two scholarships every week of break. By doing this you save yourself a lot of time during the school year when you might potentially not have the time to write a load of scholarship essays. By getting started early you also make applying for scholarships easier for yourself as you will already have a repository of essays that you can mix and match to create essays for other scholarships. Getting ahead of scholarship applications during the summer will save you more time during the school year.
Another important thing to know is that during the summer, it is completely acceptable to relax and take a break. After all that is the purpose of summer. Doing nothing is beneficial for your mental health and is a good way to decompress from the pressures of school. However summer is a great opportunity to build your resume and do some exploration as to what you want to do after high school.
-- Gabriel Moran