I advise checking hotels right up to the last minute.

I had the Marriott with high rates on one of the nights I needed but booked the Hilton early to a good rate. Then the Renesaince became available at good rate about 3 weeks out.

Then finally the day before my departure the Marriott became available at the HPR Hertz one night free rate and I ended up where I wanted after all and paid just 248 euros for 3 nights with upgrade to exec. level.

It is a tough market though. When there is a big congress the rates go sky high. www.accor.com have many area hotels but they apear as well to be quite booked in Cologne but wide open in closeby Bonn and Dusseldorf.