Perhaps one of the best types of extracurricular items you can list on a law school application is a leadership position. The more recent and difficult to obtain, the higher the committee will regard it. For example, if you were high school class president, that's good. But if you were president of your college's Young Republicans, that's better. ROTC is another great way to obtain leadership experience. Other examples include editor of a class newspaper and captain of a sports team.
The legal profession is a profession of writing. Therefore, any extracurricular activity that depends heavily upon writing will help you get into law school. School newspapers are a great place to begin. If you can achieve editor-in-chief status, you have an excellent start. Activities in which you submit essays also show your ability and desire to write well.
If you plan to practice trial law, there is hardly any better training than debate. In fact, a trial is a debate. Not only must you be able to present facts in a coherent manner, you must be able to do so in a way that makes people believe you. In addition, you must quickly formulate methods to negate an opponent's statements. A college admissions committee will attach a lot of weight to a successful debating resume.
Volunteering is always a great choice. This shows the committee that you care about more than just yourself and want to take an active part in your community. To take things a step further, try to find a volunteer position at a free legal clinic. Although these positions usually go to current law school students, you never know. The closer you can tailor your volunteer activities to the legal profession, the more weight they will carry.
If you are unable to obtain experience in one of the previous areas, don't give up. Law schools also look for diversity. Any extracurricular activity is better than none. If you have in mind a particular area of law you wish to practice, try to find an activity related to that field. For example, if you plan to practice environmental law, find activities centered on environmental issues. If you wish to work with humanitarian issues, get involved in social welfare groups. Be creative and look for something that will set you apart.