Responsibility - Give your interns something small, but important to do and make sure you celebrate their achievement of it. The feeling of not just being a momentary wisp in the history of the company is easy to give and will be remembered forever.
Ownership & Empowerment - If at all possible, give your interns discrete projects that they can take ownership of. e.g. Get them to do (initial) market research in an unexplored area = they become the office expert on that subject = empowerment.
Guidance - It’s been mentioned in other answers that the best thing about an internship is that it’s a learning experience. I don’t think this quite answers the question, as it’s a given: if your internship isn’t a learning experience already, then it’s slave labour. Volunteer to teach things. ‘Hey John, do you know about how body language affects negotiations? Here let me show you a few hand-shaking techniques.’ I promise you John would never have asked for that lesson.
Storytelling - Tell their career story for them. If you’re not planning on keeping them on after they finish their internship, then you need to send them out the door with their career story already maximised - it reflects really well on your company, as well as being something young people tend not to be too good at.
Precise and Achievable Wins - This relates to Ownership and to Storytelling. The idea is that in such a short space of time, you want your interns to go away having actually DONE something. They should be able to say ‘I did X’, rather than ‘I contributed in a minor way to X Y and Z projects’. And celebrate the wins with cake, smiles and many mentions of their name.
But, above all, show them how great it is to work in your office. You don’t want them to (a) have a bad time and (b) go away telling everyone how rubbish your corporate culture is.
Author: Will Bentinck is Business Development Manager at Enternships
Note: This is an answer to a question on Quora which you can vote for here.