In Potsdam, Germany, Rail Europe (800/622-8600, www.raileurope.com) offers a hop-on/hop-off double-decker bus tour which stops at 12 sites of cultural interest, including Luisenplatz (for its fountains), the Film Museum (€3.50) and the palace gardens of Sanssouci Park (entry to Sanssouci Palace, €8 or €12).
A pass for one circuit, including live commentary in English, costs $21 adult and $16 senior. Tours operate 9:20 a.m.-2:20 p.m. April 8-Oct. 31, 2009.