Can you help me plan 1-2 nights in Germany?

I am trying to organize a two-night trip to Germany at the end of February, beginning of March. My biggest criterion is to do it in the cheapest but safest way. Places to visit is the second plan. As far as I can see, the cheapest flights are Dortmund, Stuttgart, Hamburg, Frankfurt, but I have no information about these cities. Which cities are expensive etc. I need some ideas.