I'll have my BS in May, so I've only done co-ops, but in a wide range of fields. I originally wanted to be in the aerospace or defense field, but I couldn't get any co-ops in that area. I took what I could get.
Industries:
IT: Worked with enterprise level routers and switches.
Chassis design, thermal work, PC board layout, and cosmetic stuff(design pretty covers for the otherwise ugly sheet metal). This was interesting because it covered so many different aspects, but didn't have the cool toys aspect.
Process control:I did instrumentation on an oil refinery and poly silicone plant. This involved writing spec sheets for thousands of temperature sensors, pressure sensors, valves and other assorted things, as well as some safety interlock design. This had a lot of cross over with EE. It wasn't bad work, I actually like most of it, but I can't sit in front of a computer that long. This co-op really introduced me to the idea of potentially working the nuclear field. The office was shared with a bunch of nuclear industry people so I learned a bit about it. These were all engineers with MBAs who did consulting work for plants to analyze upgrades/new equipment and the like.
Research: Working in a research lab with superconducting magnets. This was fun, design and making the test fixtures or whatever else was needed in the lab, then actual making and testing the magnets. Academia seemed to move fairly slowly, as a lot of time was spent writing proposals and grant requests.
Future: Going into the Navy to be a nuclear officer. This is by far the most exciting field for me. It has cool toys, some amazing training and apparently awesome opportunities after your service time is up.
ME can be found in almost any field, really to the point where other people can't give advice. I have a lot of friends that are in or going into the medical field. It may be a regional thing, but it seems like new companies are all over the place and doing some really cool things.