I was stationed in Germany for 2 years in the late 70's.
Nothing wrong with Frankfurt, but it wouldn't be my first choice for vacation. Kind of a bland, color-me-gray commercial city.
If you have time to change your plans, I would highly recommend Stuttgart. It is often overlooked by tourists, but has lots to see and do. It is smaller in scale,much like Boston, and is a very inviting walking city with excellent public transit. World-class zoo, good museums, night life, parks, cafes, local and international cuisine and riverside activities on the Neckar. The surrounding countyside has lots of scenic towns with castle ruins, history, farmland and charm. Easy to visit local markets, shops and gasthaeusen. Some of the best regional beers and wines as well.
If you are staying a while, perfectly situated as a base for day trips. Zurich, the Black Forest, Strasbourg, France across the Rhine, Munich and much more all two hours or less away.