Mentoring is work and is not something one should take on lightly. You need to be willing to forge a new relationship with someone, and it is important to entrust him or her with a meaningful project. The program is not designed to just find someone to do work that isn't getting done. One other thing mentors can do is to help the interns see the larger picture of campus life, share your thoughts with them and create a safe environment in which they can learn. Practically speaking, the intern and the mentor should have regular meetings because interns don't always know when they are stalled.
Accessibility is key.
I also think it is absolutely necessary to honor the needs of the intern's home department, especially when there is an emergency or the intern has to return to take care of something. The intern couldn't relax enough to learn while worrying about that sort of thing. I am an ardent supporter of the Staff Internship Program. It is without question one of the best things the campus does for its employees.