Konstanz, that's the city I was talking about. If you like Heidelberg or Freiburg you're gonna love Konstanz, I promise you that, I don't know a single person that's been to Konstanz that hasn't enjoyed his stay, also if it's only for a day.
If you're considering a trip there, let me know, I can give you some advise on hotels and restaurants.
Traveling alone, I generally like to go out for a nice meal in very good restaurant with a youngish crowd and then like to go to a bar/nightclub, etc. afterwards. Is this the type of city for this?