Wiesbaden
From Wikitravel
Wiesbaden is the capital of the German State Hesse. Should you have to overnight near Frankfurt Airport consider staying in Wiesbaden and visit this small, pretty city. It is much smaller than Franfurt, but offers a lot.
Wiesbaden is a historic spa city, which has catered to people from countries near and far for many centuries. Even the "old" Romans knew of the "Aquis Mathiacis".