I will be in Frankfurt for business Feb 9 - 12. Meetings will be Friday and Saturday and then I have Sunday on my own. I would rather not spend it in Frankfurt. The question is - Where do I go? Suggestions for a day trip? I'm leaning towards Heidelberg though Cologne, Dusseldorf, Weisbaden and Stuttgart are possibilities as well. I'd like to keep it within 2 hours and I fly out at 11 the next morning so I need to return to Frankfurt that night. Needs to be a place that will actually have activity on Sundays. (Frankfurt I know will not.)

Thanks in advance.