Going to Europe alone this summer what cities would you recommend.?

I'm 22 and going to europe alone for 2-3 weeks and want to have a fun time.. I was thinking either going to Scandinavia: copenhagen, stockholm and helsinki... Western Europe: Amsterdam, Dublin, Hamburg.... Central Europe: Prague, Krakow, Berlin.. What region or city would you recommend?? I'm going in July so i have to make up my mind soon so i can book the flight