Trier is a 2,000-year-old Roman city near Luxembourg. It was home to six Roman emperors and has many ruins, including the impressive Porta Nigra, a four-story stone gate that was once part of the city's walls.
It is also the birthplace of Karl Marx and is a university town with a lively nightlife and interesting Franco-German cuisine. It's well-worth a day trip. If time permits, consider the boat trip up the Moselle to the neighborhood of Pfalzel to see the scenery.
110 mi/175 km west of Frankfurt.