Great question and I have had several interns work for me over the years. I think that first, internships give you the opportunity to work with engineers in an area that you are interested in. So if you are interested in design, a good internship would be in design, but working in the factory would also give you incite as to what you are designing for.
Second, a good internship is one that challenges you, let you used your engineering education, and do something that is meaningful. If you get the interview, ask about what previous interns did while they were there. This is the best indicator or what you will be doing.
It is already January, if you have not already done so, start applying. A lot of companies pick interns in January.