Freiburg im Breisgau
Freiburg im Breisgau is a student-city in the South of Germany. It is very close to the French border, and not far away from Switzerland as well. (It's the biggest Freiburg in Germany, there is also Freiburg/Elbe and Freiburg im Üechtland in Switzerland.)
Freiburg might be a good stopover for a longer trip in Europe because it's quite central and easy to exit and to leave. Another advantage is that it's easy to find free accommodation via hospitality exchange networks in Freiburg.
Freiburg is a nice student town with around 220.000 inhabitants – 40.000 of them are students. So you definitely gonna find someone to drink a beer with.
People who have the transferable monthly ticket RegioKarte Übertragbar can take along another adult on Sundays and holidays. Students of Freiburg University don't pay after 19h30 on buses trams and trains within a certain region, and on Friday and Saturday, there are trams running all night. It does not include night buses. Not so many controllers, but they appear out of the blue undercover. (year this was valid?)