Saarbrücken is the capital of Saarland, the smallest land of Germany. The city is a major industrial center but at the same time you can find some interesting landmarks there. There's no historic center, only the downtown that lies on both banks of the Saar river near the Old Bridge. It will take you a couple of hours to walk around the downtown.
In 1381 the land was called Nassau-Saarbrücken and ruled by the Nassau dynasty. You can visit the residence of the Counts of Nassau-Saarbrücken.
The most interesting building in the city is Ludwig's Church (Ludwigskirche) which you can see on one of German commemorative coins of 2 euro. On Saturdays there's is a city market near the church.
Saarbrücken is connected to the French town of Sarreguemines with the Saarbahn tram line, one of the two international tram lines in the world.
We spent the night at the Mercure Hotel Saarbrücken City (booking.com), it has the best location.
Getting there: It's easy go get to the city by train, but trains such as IC or ICE run just a couple times a day. Therefore, most likely you will take a regional train from Frankfurt, Mannheim or Kaiserslautern.