Hand write thank you letters to your supervisor and those who you worked with and who helped you. Make sure you send these within a few days of your leaving if you cannot give them immediately.
Say goodbye to everyone and send a note to those you wish to say goodbye to but cannot find.
Share your career plans with your supervisor other key people you have met. Try asking them for advice in how they think you should develop a career in the industry. Ask them how they started and planned their career. Plan these questions but make them sound spontaneous. Typically people will be very happy to talk to you and be pleased you have taken an interest in them. You may find this sticks in their memory and the time may come when they have something to offer you. A significant company vacancies are never advertised but are filled through targeted approaches. You might want to plant the seeds for such a future approach.
Keep in touch with your company contacts. You can do this in a number of different ways – through social media such as Linkedin or through occasional emails. What do you say? Perhaps commenting on a success the company has had or a recent success you have had. Keep the updates brief and don't be boastful.
Keep in touch with your fellow interns (if any). They may end up in companies that could be of use to you in the future – and you to them.