I was recently in Vienna and noticed that taking the train over to Bratislava and visiting Slovakia's capital was a very easy trek.
Trains leave every hour to and from the two cities and for about 14 Euro, anyone can afford this trip. Just buy your ticket from any automated machine at the Wien Hauptbahnhof station in Vienna.
Bratislava is small so you can easily walk 15-20 minutes downhill from the station to where the Presidential Palace, Old Town and the Castle are located.
This is a great option for seeing a unique place and making even as little as a 1/2 day trip if you're staying in Vienna.
