I'm still a student, at the moment, and am looking for jobs/internships all the time, so I probably can't contribute to this topic, but I'm interested in what jobs have been the most satisfying/rewarding to you. Even though everyone is different in what would be an enjoyable job for them, perhaps it can shed some insight for some people.
The very best jobs I've had have involved really challenging problems - the sort where you aren't sure if it can be done when you start.
It's also important to get to know the workplace culture where you'll be working. Having really talented people is a joy. It's also very helpful when the culture is on fixing problems, not assigning blame. My current company does this very well: we all recognize that everyone is human and makes mistakes. When something goes wrong the emphasis is on finding the problem and fixing it. Nobody is worried about CYA.