Goethe's house
Ever hear of Marlowe's "Faust"?? If so, then you have also heard of Goethe, who originally wrote the famous story. Though well known in Europe, he is not a common name abroad amongst those who aren't interested in literature. The house itself is amazing, and is stocked with many of its original furnishings... Goethe's original birth certificate even sits mounted on the wall in the room of his birth! The library is also amazing. It truly is a step back in time, though I would advise taking a translator of some sort along with you if you're not fluent in German. There is a map of the house written in English, but everything else, from the museum downstairs to the captions that describe items, are completely in German.