Freiburg is a really interesting place, with its liberal and evironmentally friendly attitude combined with interesting inner city. It reminded me of San Francisco, except smaller and much less gay. I did manage to get a little bored here by the end of my three days, so I only recommend two tops. Going to see a match …
Konstanz is amazing. I love this place! I want to come back.
Heilbronn is a nice, medium- to small-size city with a little bit of everything to do. I've never stayed in a hotel there but it's a nice town/city. Summertime offers nice festivals and the wintertime offers the beautiful WeihnachtMarkt! Enjoy it!
Hohenstaufen is a interesting town. I love to visit, especially since I have relatives there. There are friendly people and good shopping in Göppingen.
Karlsruhe is a great midsized people, about 250,000 people live there. You can find a huge palace and many cultural things to do. Nightlife is ok, too. And Karlsruhe is one of the warmest cities in Germany very close to the Rhine River, the Black Forest and the French Border (all within 30 minutes).
Baden-Baden is fun to explore especially with all of the staired ally ways that lead into seemingly secret courtyards with foutains. Bring a good pair of shoes because the town is very hilly.