Norbert,
dunno. I won't make it for the Camp, unfortunatley.
ESUG has a link to HRS which shows all hotels sorted by distace from the venue. I have been staying at the Lindner Hotel City Plaza (too expensive for me this time) and can recommend it. I've heard of a few of us staying at the Motel one. If you'd like to have a pool and sauna in the evenings, I can recommend the Maurittius Hotel & Therme. The rooms aren't really great and you may have bad luck and hav loud water cleaning or heating in front of your room, but other than that, it is nice.
I also know the ibis koeln city center, which is a typical ibis, good to reach, nothing spectacular.
If you'd like a spectacular breakfast, I can recommend the Cerano Hotel Köln City at Elisenstraße.
Looking forward to meeting you in Köln!
HTH
Joachim