Stroll the pedestrian mall, drink gluhwein bought from one of the outdoor vendors.

Do not, repeat do not remark that you really don't like Kolsch bier in a crowded bar, unless you want to relive the scene from Frankenstein where the villagers came out with pitchforks and torches. (Not that I am speaking from personal experience of course, and on a completely separate note, one day I hope to be allowed to return to Koln.)