Which group of cities is on the Danish peninsula?

Schleswig-Holstein is the northern-most of the sixteen states of Germany, on a peninsula capped by mainland Denmark. It is often referred to by its Danish name, 'Slesvig-Holsten'. The dual name is a composite of two historic duchies. A small fraction of the state abuts into the German mainland bordered by the states Hamburg, Lower Saxony and Mecklenburg-Vorpommern (Upper Pomerania) in west to east order.

Kiel is the state capital. Located on a bay subset of the southwest Baltic Sea, Kiel is a major maritime center of Germany, hosting many sailing events, most notably that for the 1936 and 1972 Summer Olympics. It is also a long-time home of the German Navy's Baltic fleet and high-tech shipbuilding.

Lübeck is the historic birthplace and capital of the Hanseatic League, a medieval commercial and defensive confederation of merchant guilds and market towns in Central and Northern Europe.

Flensburg (in image) is very close to the Danish frontier; Odense, near on the other side thereof, is the birthplace of the fairy tale author Hans Christian Andersen. It is Schleswig-Holstein's third largest city in area after Kiel and Lübeck.

Travemünde is a borough of Lübeck, located at the mouth of the river Trave in Lübeck Bay. It began life as a fortress built by Henry the Lion, Duke of Saxony, in the 12th century to guard the mouth of the Trave. It became a town in 1317 and in 1329 became part of Lübeck. The fortress was demolished in 1807.