A friend of mine goes to Filderado (this is by the airport). This is the website:
http://www.fildorado.de/ (you can try to translate using babelfish)
The SI (Millennium) and Pullman (formerly the Fontana) also have spa services.
If you are interested in the Turkish Hammam, both the Millennium and Filderado offers that service.
If anyone is interested in a bikini wax, the Millennium does not provide that service, the Pullman does.
Hope you find this helpful!