I've always been fascinated by 1913, the last year before the Great War, so many dynamic inventions and social changes underway, and yet so different with the monarchies, the various colonial empires etc. If I could go back in time as a tourist and could visit just one country, I think I'd have to choose the Austro Hungarian Empire, From Dubrovnik to Cracow, from Bregenz to Lvov (Lemberg), what an amazing assemblage of beautiful cities, nationalities, religions, cultures and countryside, all somehow muddling through politically, though obviously cracks were showing that would emerge starkly in 1918.
Just the natural beauty alone, the Dalmatian and Istrian coasts, the mountainous regions, Alps, Carpathians, Sudeten mountains, the cities of Prague, Vienna, Budapest, etc etc. The fashionable resorts like Karlsbad, Bad Ischl etc,, Don't know how efficient and tourist friendly every thing was, like railroads and urban transportation, but it must have been an amazing place to have visited at that time.