A great travel strategy is to concentrate on major European cities for the winter months.
In 2012, I spent an entire week of January minus one day in Paris. Landing on New Year's Day, I felt that I had the City of Lights mostly to my self. The crowds at major sites like the Louvre were small. Most of the sites were open though on reduced hours. The restaurants were moderately filled or empty; the waiters at one eatery in the Left Bank seemed almost tickled I had walked in. The city noise was low. The Metro wasn't packed shoulder to shoulder.
Just for some variation, I stayed in two different hotels. And to get out of the city for one night, I took a one-hour train in Chartres and spent the night, enjoying the pilgrim cathedral and small town without the stress of summer.
Friends and family couldn't believe I'd go to Paris in January because of the cold. In that, I might be blessed to be from Chicago. in comparison, Paris felt balmy -- about 10 to 15 degrees warmer than the Windy City. Someone I met in Paris said February is an even better month to visit because January nets some people who have days off for the holiday.
This strategy can apply to any major city -- Berlin, London, Madrid and anywhere. The tourist hordes are small in winter, and the usually high stress level of a big city in other months is much lower.