Zagreb is the capital and largest city of Croatia, which was also part of the former Yugoslavia and has similar historical ties with Slovenia. It has a population of about 800,000, which is close to Ljubljana's 300,000.
Tallinn is the capital and largest city of Estonia, which is also a member of the European Union and has a history of being under various foreign powers, such as Sweden, Russia and the Soviet Union. It has a population of about 440,000, which is comparable to Ljubljana's.
Vilnius is the capital and largest city of Lithuania, which is also a member of the European Union and has a history of being part of the Polish-Lithuanian Commonwealth, the Russian Empire and the Soviet Union. It has a population of about 540,000, which is similar to Ljubljana's.
Riga is the capital and largest city of Latvia, which is also a member of the European Union and has a history of being under various foreign powers, such as Germany, Sweden, Poland, Russia and the Soviet Union. It has a population of about 630,000, which is close to Ljubljana's.
Helsinki is the capital and largest city of Finland, which is also a member of the European Union and has a history of being part of the Swedish Empire, the Russian Empire and the Soviet Union. It has a population of about 650,000, which is close to Ljubljana's.