Karneval in cologne. get drunk and party with two million people in the streets and in every pub or club.
feb.4th is weiberfastnacht, the official day of the year where it is mandatory for every girl to become a total slut, but in a good way.
rosenmontag on the 8th is the second best day.
inbetween those dates you can either visit the festivals at all the small villiges around or visit historical sites which are all over the place.
I highly recommend a hike through the siebengibirge
and visit a bunch of ruins, even from the siegfried saga and snow white, castles, wwII ruins like a V1 launching site, old chocolate chip cookie bunkers, weird ruined mansions in magical forests. in the winter, if you are lucky, you might even find ice forrests.
pics of places you can all visit in a one day march in the siebengebirge:
alternativley you are welcome to visit me in my new home, bavaria. but only if you are into snow, we just got a lot of that. I live just around the courner of newschwanstein
also, germany and austria have a great tradition of drunk skiing and partying, so you are welcome to join me in that as well ^^
last but not least, I highly recomend sothern spain as well. fly to malaga, rent a motorbike and travel east : )