Less than an hour away, Bratislava is a very accessible and interesting city for a daytrip.
Popping across the border for a visit to another country.
A very common, interesting, and affordable daytrip from Vienna is to the capital of neighbouring country Slovakia, less than an hour away by bus or train, a bit longer by boat on the Danube. Bratislava is Europe's smallest capital, with barely 500,000 people, making it very easy to take in with a single day's visit. St. Martin's Cathedral, St. Elizabeth's Church, and the Bratislava Castle are the three principal sights not to miss in the city's old town, and that still leaves plenty of time to get to St. Michael's Gate, the only one of the city's four medieval city gates that remains.