What are some good tasks to assign co-op students/interns? [closed]

Question

My company is investigating hiring a University Computer Science co-op student (BSc year 3) for a 4- or 8-month work term. (I'm not sure how internationally-recognized the term co-op is - it's essentially a paid internship, after which the student returns to their studies.) My team develops a web application in ASP.NET and handheld thick clients on iOS, BlackBerry, and Android.

For any of you that have brought interns into an experienced development team, what kind of tasks did you find for them to do? I realize that's a hard question to ask since any answer can be quite specific to an individual business.

I'd expect the following:

• Tasks that require mentoring but not babysitting
• Tasks that will take a few weeks to finish (so the mentor can remain productive)
• Tasks that expand the student's understanding of software development

I've considered things like expanding code coverage in our unit tests, or developing a feature that's been designed, or improving/writing missing requirements documentation from features that were added without any supporting documentation.

I never did an internship so I don't know what sort of tasks are valuable to both the student and the company. Any recommendations?

Edit: These are all excellent and thoughtful answers that have helped me a great deal. Thank you all very much. I can only choose one answer, so I'll go along with the crowd and accept the most popular one.