Groups - Mock trial or moot court, pre law society, and student government are all valuable. Also volunteering. I volunteered with Volunteer Income Tax Assistance (VITA) program preparing income tax returns for low income people during tax season (January through April). They trained me to prepare the returns and I helped clients by preparing their returns during sessions.
Classes to take for your resume - Law classes and those that teach you reading, writing, and researching skills. Reading, writing, and researching are the essential skills that every lawyer needs.
Jobs - You could try to get an internship at a nonprofit organization that develops public policy while in college. Look at your interests - for example, do you care about the environment? You can look at the Sierra Club and other types of groups that focus on environmental issues. First you want to think about what you care about and then go from there. Or look at what organizations are in your area.
Important tip - make sure you don't take on too many activities and focus on your studies. Your GPA is an important factor that law schools look at so make sure you study and keep up your GPA!