Crystal Cruises (310-785-9300; crystalcruises.com) is one of a number of lines offering Baltic Sea cruises that make stops both in St. Petersburg and German ports that offer excursions by bus into Berlin. Stops on a seven-day cruise beginning and ending in Copenhagen include Warnemunde, Germany; Helsinki; and St. Petersburg. Cabins: $3,895-$10,455 (accommodations range from basic window stateroom to penthouse suite). Cost of additional excursions to Berlin and St. Petersburg: $69 to $1,029 (depending on scope of excursion chosen).