I've done Ruby and Cataract at Thanksgiving, the Goosenecks on the San Juan over Christmas and the Upper San Juan in January. Just watch the weather and get beta about ice dams before making plans, but these sections are less likely to freeze than some others.
"There is NOTHING--absolute nothing--half so much worth doing as simply messing about in boats. Look here! If you've really nothing else on hand this morning, supposing we drop down the river together, and have a long day of it?" -Wind in the Willows