The Lutherhaus is the largest surviving residence of the Reformation era and now houses a substantial museum on Luther's life and the development of Protestantism. The building itself — a former Augustinian monastery — is as interesting as the exhibits: you can walk through rooms where Luther lived and worked, including the study where he translated the New Testament into German.
The museum collection includes original manuscripts, early printed editions of Luther's writings, paintings by Lucas Cranach the Elder, and objects from Luther's household. The explanatory material is thorough and available in English. Allow at least 90 minutes.