Bratislava, capital of Slovakia, is a 1 hour train ride from Vienna. The distance between these two capital cities is suppose to be THE shortest distance in the world but I have read contradicting evidence so I don’t believe it. The close situation of these cities means a great day trip from Vienna. Take bus X13 or 93 to get closer to the Old Town. If you buy your Bratislava ticket from Vienna, the ticket includes free public transport usage in Bratislava.
The Old Town can be easily covered in a day plus maybe a museum as well. Taking a “hike” up to the castle grounds will reward you with sweeping views over the Old Town, Danube River and the ‘burbs (suburbs).
It’s a small Old Town but many different buildings to feast your eyes upon. There are plenty of outdoor cafes, cute stores and amazing historical buildings although some are time-worn.