When you first decide to take an extra class you may wish to begin with one that is related in some way to your curriculum. For instance, an English major may consider a refresher computer course that will allow him or her to work quicker when it comes to term papers. Another possibility might be something related to researching documents—these may not be part of your curriculum but will help you achieve your goal of obtaining your degree in your field of interest. However, you should first fill in your time with those classes that will help you obtain your degree—save non-degreed courses for those semesters when you have some extra time to fill.
Students that are commuting from home to college or those who choose to stay on campus for the summer may find that summer is a good time to take some extra classes. Some students choose to go to college all year in order to graduate sooner, but during the summer there are not as many offerings as there are in the fall and spring semesters. This doesn't mean you should forego taking any classes during the summer because these extra classes will help you keep your mind working at top speed, much like during high school when you have all summer off and forget things by September. Keeping your mind fresh by taking even one class during the summer semester will keep you energized.
You can also just take a class that you don't have the time to take when you have your full-time schedule running. Summer can also be a time to look at your options and see if you are meeting the goals you set for yourself in terms of progression toward your degree. Maybe you have some areas in which you are still weak though meeting course requirements: you have some time to brush up on those skills and make improvements without the stress of passing a class.
Since you're already in college anyway if you have time open in your schedule you may want to consider taking a class just for the fun and relaxation of it. Sometimes having something you enjoy doing helps take some of the stress off of all the studying and researching that is necessary in the pursuit of your degree. Taking an extra class just for enjoyment, however, should not interfere with the time necessary to complete assignments in your regular classes. What you want to accomplish by taking a class for fun is just a few hours to relax and take the stress off the requirements of your regular credit classes.