If you're looking for European quaintness, it's hard to do better than the pretty German town of Bernkastel. Located in the heart of Moselle wine country, Bernkastel specializes in dry white whites made from the Riesling grape; if you think all German wines are sweet, you'll be pleasantly surprised by the quality in this area.
Even if you don't like wine, Bernkastel provides a pleasant stop. The old town is full of half-timbered buildings from the Middle Ages, including the Town Hall, which dates back to 1608. There are also cafes and restaurants for people watching or a late lunch, as well as shops for souvenirs. Those seeking a nice walk can go up the hill to the ruins of Castle Landshut, which offers nice views of the valley; there's also a shuttle bus from the village. If your riverboat has bikes onboard, there's a path along the river for riding. Finally, Bernkastel is occasionally used as a jumping-off point for day trips to Trier or Luxembourg.