1. Thank people. Talk to your manager about what you got out of the experience, and thank her for giving you the opportunity to work with her. People love hearing this sort of thing—don't be shy about telling her. Thank other people you worked with as well, especially anyone who went out of their way to be helpful to you.
2. Ask your manager for feedback. You want to know her assessment of your strengths and weaknesses, because that's valuable information that will help you do better in the next job. Say something like, "I'd really value hearing your advice on what you think I did well and what I could work on improving."
3. Talk to people about your future plans. Let them know what you're hoping to do next summer, or after graduation. Tell them you'd love any advice they have, either now or in the future. These people might be quite helpful to you in the future--telling you about job leads, recommending you for a job, helping you figure out career choices, and so forth. But a lot of people won't offer this kind of help if you don't explicitly ask for it, although they'll be happy to help if you ask them to.
4. Update your resume. Now is the time to add this job to your resume, while the details of your accomplishments are still fresh in your mind.
5. Reflect on what you learned from the experience. What do you wish you'd done differently or known when you started? Can you see yourself working in that field? Would you want to do the work you saw others doing? Was the culture one you'd like to work in again or try to avoid?
6. Keep track of contacts so you can stay in touch. Once you're back at school or in another job, send the occasional E-mail updating your boss and coworkers from this internship on what you're up to. Very few interns bother to do this, but those who do really stand out—and often develop professional relationships that serve them well long into their careers.
Alison Green writes the popular Ask a Manager blog where she dispenses advice on career, job search, and management issues. She's also the author of Managing to Change the World: The Nonprofit Leader's Guide to Getting Results and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development. She now teaches other managers how to manage for results.